I think the age check requires a PIN, noone but the owner of the card is supposed to have it. Dunno whether the check shows up on the transaction history it probably should.

Germany is actually kinda strict about age checks, it's the reason why not a single porn site is hosted here they'd all need that level of age auth. OTOH it's also understood that kids will find ways to circumvent things and that's also fine because you can't stop them anyway, if they do so sensibly you can ignore it and if they're not being sensible you can whip out the good ole "I'm not mad, I'm disappointed", either way they learned something about responsibility. Learning to keep your parents on a need-to-know basis is a rite of passage.