From what I think I know, they leave the fry to fend for themselves.
In the wild, the fry hang around the roots of floating plants (such as water lettuce) for security against predators. As they grow into mini looking piranhas, they will shoal with other non-piranha but look-alike fish (such a silver dollars and pacus). This way, they get the security in numbers, but have a food source (just fins and scales at this stage) living along side them.
Thought I would share that little information I have.