Confiscate the hell out of it. That's what parenting is for. How much money is a kid going to spend on burner phones before deciding to just stop bringing them to the house?
I don't really understand what you're arguing for here. Obviously prisons understand they can't catch everything, but they try anyway because it's still better than letting prisoners bring in whatever they want.
They try, and they fail comprehensively, and that's despite being very willing to do things that would be extremely clear child abuse if I tried them on my kids.
The prison warden doesn't care if the prisoners love him 20 years from now.
$50 (or a hand-me-down from a friend) will buy you an Android burner phone that can hop on the neighbor's wifi.