Twitch’s Terms of Service require users to be at least 13 years old. Even then, if they’re under 18, they’re supposed to have adult supervision.
Pro tip: When my kid ventured into similar platforms, I used Parentaler to monitor their DMs and interactions. It gives you a good overview of who they’re talking to without having to look over their shoulder 24/7. It’s a solid way to respect their space a bit while still ensuring they’re safe.