So. Today I was sitting on the floor admiring my budgies. Mum eating lots of millet and then popping out to see me. Dad sleeping up top. Oldest baby eating some seed and being fed my mom. Youngest exploring outside of the box and in his little igloo. Then out of nowhere. Oldest baby starts to feed the other!!! Is that normal?? What's up with that?

Yes that's normal; at one level Budgerigars are very sociable birds. It seems that only at breeding time do they get aggressive to others as a survival method, but at other times they are very sociable. Feeding others 'their mates" is very common. I'm pretty sure that even in the nest siblings probably exchange food, but it's difficult to observe. I have noticed that siblings at weaning time often feed their young siblings, even when they are only just learning themselves. I just lately had some young that were deserted by their parents, they all survived, even one that had it’s feathers just opening. I witnessed that they were helped, by other young birds in the aviary, who had only just learnt themselves to feed, but were unrelated to them.

It's pretty common for the older chicks to help feed the younger ones.

Well that's super awesome! It's really cute to see!