I found it:

thank you! yes, this is logical. I don't think they would hurt the baby without reason.
I feel guilty because I could give him some Gatorade and I didn't. His only problem might have been being born a few days later than the siblings. his skin looked red and he was making groaning noises but parents were with them a lot, so I thought they would resuscitate him.
I will change bedding today a little bit later.

I also thought to take young birds out as we having a very nice warm sunny day.

I cleaned the box (actually just replaced bedding - didn't want to keep little ones out for long). I haven't found any flat chick or anything except poop but I will look again (I kept bedding in the bag)
The chicks look all right, with full crops. Weights are 30g, 23g, 22g, 14g, 12g. None opened his eyes but last time they opened close to 10 days, not 7.

it's a beautiful day (sorry to say this when I lost a baby) but it's not easy to take birds out (too bad they are staying indoors!). I got one of the young birds (a tame one) and put her in the cage. Then others started to fly from me like crazy. Parents who were eating took off like crazy too. Of course, the one in the cage was flapping her wings like crazy while others were flying. For babies' sake I stopped my attempts. Ridiculous, its 70F today but inside the house it feels chilly (its strange to run heater on a day like this). I wish they all could go outside.
After cleaning the box, all the babies are scattered and not cuddling together anymore. This is what I don't like about cleaning.

I haven't found a flat chick or any dead chick when I looked once again thru the bedding. Does it mean they ate it? Do they eat babies sometimes? I think I read about it...

It's not likely that they ate it, but sometimes when they've flattened a body it's very difficult to identify it. Is it possible that you miscounted the babies somehow?

If the babies aren't clumping together now it's probably because they don't need to. As they get older they can maintain their body heat better and don't need to stick together for warmth.

Not possible to miscount. They had total 8 eggs. Then 5 chicks hatched one after another, I removed all egg shells and we had 3 eggs remaining. Yesterday I saw a broken shell and there were 6 babies. One was much smaller than the others. I removed his shell and tried to free his body from siblings who were on top of him.

I candled today 2 eggs and they are not fertile (thanks God). Also, the babies are not leaning towards this eggs anymore.

Another relief is that parents feed the babies - no problems with this today, father even more often than the mother (well, she stays with them overnight), so no need for older chicks to go away. They looked pretty stressed yesterday when they spent a big chuck of the day away from parents. One little singer ever stopped singing and they all looked sad.

If the parents aren't interested in the eggs, I think it would be best to remove it to prevent any bird from accideny breaking it.

thanks, I will remove the eggs.
I have a feeling that our cock just needs some time to catch up with his responsibilities. this is the second clutch for both of them.
Is there still a big need for additional calcium?

do you know if its still possible to access SR_tiels pages?

I definetely need to train all the birds to make them more manageable.

Another question. If the babies don't need to be fed every 2 hours now (or even if its every 2 hours), why do parents go into the box more often than every two hours and I can hear feeding sound?