Thank you.
I am very frustrated with the situation. Here are the problems

1- Mom started to lay eggs (3 by now) and she is very quietly sitting in the nestbox all day long. She doesn't even eat. She comes out to eat in the morning to the kitchen table (where I used to feed them when they were feeding first clutch) and basically this is it. She used to be craving mineral block - not anymore. I am not saying about veggies, maybe a tiny peace in the morning. I guess she comes out a few times during the day to drink some water and maybe eat some seeds (this is all she can find in the cage).

2- Dad is being..... hmmm... I would call him a jerk, or he is simply not very intelligent (sorry, Trillie). He is not eating much as well. Just because he copies that the hen does, and she is not eating. I really doubt he has anything to feed the chicks.

3- chicks are losing weight and I am very concerned. For example, one lost 4g during the day today (evening weight vs morning weight), another one lost 2g, two others showed the same weight as in the morning.

They have lost a lot compared to the peak weight at about 3 weeks. Bella was 106g at peak, now she is 86. Stella was 95g, now she is 79. Our younger chick, Tator Tot, was one of the biggest (103g) - now he is the smallest at 78g. Sammy was 96g, now he is 79g. Is it the way it's supposed to be??? The chicks are 4 weeks old.

Now how they spend a day. In the morning dad goes to eat to the kitchen with mom. Then he pretends he doesn't know this chicks. He may feed some of them a little bit Or pretend he fed. Then they all fly, chasing him and begging for food. I put him in another cage today and took him to the basement during handfeeding time so he wouldn't distract them. It didn't work. They are still not eating formula. I shouldn't force chicks eating, right? The chicks don't look at nice as they were. The feathers are dirty from the formula that I try to clean but can't clean completely. I am concerned I am blocking their noses with formula that dries there. They just don't open their beaks and some don't even stand still.
My family told me that he fed them "a lot" while I was gone, but I doubt he would have a lot to give them. Then in the evening we have another lovely "flying all over" game, and this it when I am trying to feed them. He is escaping from them and they are trying to chase him. Our cock isn't tame so I can't just put him in the cage and take him down. They go to sleep after 10pm when we are finally done with everything (hungry, I bet)
What is also surprising is that chicks aren't really trying to do much. Probably saving energy to chase the father.
In this situation, is it better to separate the chicks completely? This weight loss isn't normal I guess. They also look and feel much smaller that they were.
What can I try ???

I'm concerned about the weight loss of the chicks. They are chasing their father because they're hungry, and they're not eating well for you (which is not surprising because it's very difficult to teach babies to handfeed at this age). If you have access to an avian veterinarian, it would be best to visit them ASAP so they can make sure that the babies' weight isn't falling dangerously low. They may also know about some helpful resources, like an experienced handfeeder who might be able to help you feed them.

Thank you. The only experienced avian vet that I know about is very far away, more than an hour drive. We also have a bird store in the city and they buy chicks for future sell when they wean. I am pretty much sure that our chicks would start eating formula if there is nothing else to eat.

I am posting on another forum too, they have some experienced people there.

Does the bird store handfeed the babies themselves? If so they might be able to help with your chicks. Or they might be able to hook you up with a local breeder with handfeeding experience.

Yes, the store handfeeds babies and they accept them at age 3 weeks. All babies are in the nursery in cages, getting fed several times a day all together, aren't handled other than this, are not let out to fly and not given anything except formula and some pellets/seeds. Sure, its better than starving to death. I can sell them my babies.
Do you mean that they would send somebody to me house to handfeed in my house?

what you think of separating them from the parents? When they realize that no other food will be coming, won't they start eating? my feeding is not that bad, I think I would be able to feed the bird who is wishing to be fed.
I regret that I don't allow them to sleep in the nestbox, they must be weak. Especially I am worried for the one who is the youngest.

If I need to transport chicks, how do I do it in the really cold weather that we are having?

You would need to talk to the store to find out what they would be willing to do to help you. Maybe you would need to sell the babies to them, or maybe they will help you feed them and show you how to do it more effectively.

If the babies are getting weak they need to get some food as soon as possible, so talk to the store as soon as you can. If they become too weak they will not have the strength to eat. If you are able to keep the babies instead of selling them to the store, you can separate them from the parents so they will rely on you for food and will eat well for you.

The best way to transport the babies in cold weather is to put them in a small box so they are close together and will help keep each other warm with their body heat. Wrap the box in a towel or small blanket to help keep the heat in. Make some small holes in the box so the birds can get air to breathe of course, and spend as little time as possible in the cold air.

The lady I need to talk to isn't in the store today. I left her a note. You see - the situation is a little ambiguous, they were offering to buy my chicks some time ago but I was stubborn. I was also asking on forums if I needed to interfere in feeding some time ago, the answer was no. I wonder if this situation could be foreseen and prevented.
I have a feeling we are not doing that bad though (I was afraid somebody died overnight but they all looked OK). After I fed the smallest chick this morning, he measured 5g more than before feeding.
The chicks are cleaning themselves, picking up small food on the floor, etc. Sure they still need more nutrition. I will keep monitor their weight and keep feeding them.

Is it better to feed them in environment they are well familiar with, or an opposite? I took one chick downstairs today to feed as I didn't have time to chase the father downstairs, and he had no place to run away from me as he was put on the table he hasn't been before. I still had to force him eating but he ate some food.

I wipe their noses right away if I see food in the nostrils, but it still gets stuck inside. Is this dangerous or not?

I want to put it clear. I don't mind selling the chicks and I am not going to keep them for long (I just can't). I just wanted to give them more time to learn from the parents what they can, to learn how to fly, etc.
One chick got inside the nestbox while I was gone to class and mom doesn't mind.
I wonder if I should give them similar nestbox to spend a nite ... Is it crucial that they sleep on the perch? Some prefer to sleep on the floor.

As long as the parents were still feeding the babies well everything was fine, and there was no way to predict this. First-time parents don't always know what to do. At this point the father should be the one primarily taking care of the babies while the hen tends the new eggs, but he obviously doesn't understand his job.

But it's possible now to predict that this might happen again with the new clutch, so it will be good to start co-parenting the new babies at 2 1/2 to 3 weeks of age, so they'll be familiar with handfeeding and you can take over all the feeding if mom and dad don't do their job correctly.

The best place to feed the babies is the place where they will eat the most. If seeing the father is a distraction, then take them where they can't see him. Offer food to them frequently if they haven't had a lot to eat recently. Right now they're still learning that they can get food from you, but when they understand this idea better they will probably take more at each feeding.

The best place for the babies to sleep is where they are comfortable and where the new eggs have the best chance of staying warm. If having the babies sleep in the nest is the best way to achieve both goals then let them sleep in the nest.