Using money as a gate has some issues. If you don't scale the amount to the region, you effectively exclude that region. But if you do scale to the region, it encourages the spammers to simply setup their operations (either physically or virtually) in the cheapest region (physically also has the benefit of cheap labour).
Requiring unique credit cards could mitigate that, but then there is PCI issues there, and its basically no different to requiring ID.
Be a parent, and be present when they are using it. You don't have to watch constantly, but at least be nearby. Teach them basic digital safety, literacy, and privacy skills.
You cant control all the devices they have access to, so you have to give them to knowledge and tools to be safe.