There's a free app called Famigo Sandbox which will provide your kid with their own launcher with a subset of apps you've approved for their use. As well as in-app purchases, it also prevents making phone calls, sending texts, mucking about with your settings etc. and blocks ads until you unlock with a swipe pattern to get back to the full phone.
It's quite handy. I assume they make money via recommending kid focused apps to their users.