First of all welcome to PFury
I'd add a little decor for the little dude's sake: a couple of plants, rocks and/or a nice piece of drift wood.
Baby piranha's are on the menue of just about anything Amazonian that walks, crawls, swims or flies, so it's only natural (pure instinct) they rush to the nearest bit of cover, and stay close to that.
After you have added some decor, they'll probably remain close to it for a while, but over time they'll start to feel more confident and secure, and will venture out more often and for longer periods of time, until they feel like they own the tank...
As far as food goes: there's plenty of things to feed them, like standard fish flakes, frozen osquito larvae or blood worms, tubifex, krill, pellets (if the accept it). Once they get bigger, you can add things like shrimp, mussels, squid, earth worms, and fish (fillet or whole frozen fish such as smelt).
Good luck