but then how do you pay for things if/when you want to buy the little one something?

you either add the card then remove it, get a separate card, or find a way to use credits somehow -- but in the case of the latter two, Junior could still slam a few buttons and eat up all available funds on that card -- just without the impact of your regular card.

This is something that Apple really got right. My kids can initiate any kind of purchase they want (At least, those that are allowed based on Screen Time controls) but every purchase stops and waits for approval from me or my wife. Even if the item is free. No faffing about with credits, or extra cards, or setting limits.