I agree. That can also be called "common law" in Germany. Contracts with people who are not entitled to sign contracts on their behalf are not considered legally binding.
In that sense, it shall be treated as if there hasn't been a contract at all. The process is purposefully designed to get a signed contract as fast as possible. The technology to make proper ID (Age) verification is available, but my understanding is, that it is not used by facebook and its partners.
COPPA doesn't require foolproof age verification IIRC, otherwise the regular Facebook app and nearly every app on the planet that just asks for a birth data with no verification would be illegal.
In that sense, it shall be treated as if there hasn't been a contract at all. The process is purposefully designed to get a signed contract as fast as possible. The technology to make proper ID (Age) verification is available, but my understanding is, that it is not used by facebook and its partners.