If they are both greys, I can't tell the sex...sorry

Are there any small red dots on the skin in the bald area? If there are, that's a sign of plucking. It's hard to be sure but it looked to me like there might be a couple. If there aren't any of these marks it's probably plain old baldness. This area is the last part of the body to feather in, so it's possible that the feathers will come in before too long.

If it's a permanent bald spot it was probably inherited from the father, but this doesn't tell us anything about the baby's sex. Baldness in cockatiels isn't related to their gender.