I'm not familiar with piranhas, but generally for preds, 3 small meals in one day while they are babies is better for them than 1 large meal in once a day.
Bloodworms are good, but try to feed the beefheart sparingly. AFAIK, fish have a hard time digesting terrestrial proteins. Try other seafood such as cut prawn or shrimp, any market fish, squid or silversides, and try to get them to accept pellets as well.
and earthworms
Once they are larger, you can feed them every other day, or even every third day