This would be a significant obstacle if we were talking about children participating in a complex transaction, like buying a house. Such transactions involve obligations on all sides, and a lot of trouble if people don't meet their obligations. Internet access provision is not such a transaction. If I walk into the candy store and pay 75 cents for a pack of gum, I'm not asked to show identification or sign my name to a contract. I hand the storekeeper my money, they hand me the gum.
Society has no problem with children participating in such transactions.
If I don't pay, the shopkeeper simply doesn't give me the gum. Easy to handle. Internet access is similarly easy. I can misuse my account, or fail to pay for it. In either case, the ISP can cut off my access.