We have our children pay us using a card payment terminal. Ours is from SumUp, I know plenty of other brands do them too. I'm happy to eat the transaction costs for the ease of use.

A terminal should work with any product that functions as a debit card, pre-pay or not. As it happens, my children do have actual current accounts behind the cards they use. But it seems like UK banking is becoming less willing to issue full-spec debit cards to younger teens :(.