Where I live (and as the comment pointed out, laws vary from place to place), children may own property. And it is the right and duty of the parents to raise their children, which may mean controlling what the children do with their property.
- If I give my niece $100, she owns that money.
- If she wants to spend it all on ice cream, her parents may forbid that commercial transaction. They can take that money away from her, for safekeeping until she reaches her majority. Or more likely, to disburse it in smaller chunks that do not lead to stomach problems.
- If I give her $100 worth of ice cream, storage for a couple of years is unrealistic. But the parents can still control if and when (and how much) she gets to eat.
I guess your hypothetical child would be older than the niece I've been talking about. Again the parents would have a role to play.
- The parents' signature would be required on any contract involving recurring payments, like most phone contracts.
- While the parents should educate the minor about living in the real world, which usually involves having a phone, they are also supposed to supervise media use and to keep harmful content away from the minor. This may include taking the phone away, as long as they return it by the 18th birthday ...