I'm so sorry for your loss!
This does not sound like bumble foot. Especially since the budgies in question are so young. When the vet told you it would cost $400 to operate, did they tell you what could be wrong? I don't see why they would operate on such a young bird. It couldn't possibly be tumours since they only just fledged.
If you haven't already, I think you should get these babies to a Certified Avian Vet. This does not sound good.
Any chance you can post photos of the lumps?