I am not an attorney, but it is my understanding that a contract with a minor is unenforceable (that’s why parents also have to sign all those forms) so it is against the terms of service for a person under 18 to have a Paypal, Venmo, etc. That’s not to say kids don’t do it, but do not allow your kids to leave any money in a Paypal account etc. Paypal can and has shut down the accounts of adults who started a Paypal account when they were underage for breach of contract and taken all the money sitting in the account.
That said my kids both had Paypals when they were under 18 just because we didn’t read the fine print and weren’t thinking it through. My D22 does a lot of stuff on Depop which only operates through Paypal, but she knows not to leave any money sitting in her Paypal account.
These might be informative reading: