The "under-18-year-olds can't enter into contracts" laws are, broadly speaking, derived historically from child labor protection concerns, and tend to be state-level law.
There's very real risk that legitimizing a contract with a child opens Google up to expensive-to-litigate criminal liability.
Children technically can enter into contracts. But they're voidable contracts, so it's essentially no upside and only downside for those that would contract with them.
What liability is that? Genuinely asking because I've never heard that concern.