> Can a kid set up an AWS account?

Yes

> Are there no checks?

No

>Wouldn't the contract be void for anyone underage anyway?

Typically not

If a child goes through the checkout at the grocery store with cash, can the parent march in and demand a refund because "he's underage so the contract is void"? A credit card was used. Why should aws care about the details? (Other than the potential for the card to be stolen ofc.)