My budgie is plucking off the feathers off it's young one and eating them. She has always been a good mother.

Question

Why is she acting so strange ? The little one is too young to feed itself.

Answer 1

If the babies are old enough to leave the nest she may be trying to get them to go. I have seen this behavoir in budgies and cockatiels when the pair want to start nesting again.
If the chicks are very young she may be short of some of minerals in her diet.

Answer 2

You should call a vet and find out what to do. I hand raised a few baby birds and it can be done but it is a lot of work.