I don't know anything about whatever testnet wallet may be in F-droid, but I do know Padawan wallet: https://github.com/thunderbiscuit/padawan-wallet

However, while I do like a testnet wallet for learning and becoming familiar with everything, once your kid understands what's going on, just give them real Bitcoin. If they lose it, the pain of losing it will teach them the same life lessons that the pain of a kid losing their money they earned always does.