"I'm with the keeping inside the law brigade."
What law? It's a Ts and Cs issue between two civil parties. The venue in the Ts and Cs is Santa Clara, and although there are solid reasons to believe that the current UK/USA extradition treaty is a very bad thing, even its writ doesn't run to extraditing minors to California to face civil actions.
The only reason that clause is the Ts and Cs is because under COPPA, US residents who are under 13 (and their agents, who for practical purposes will be their parents or guardians) have additional rights against people who collect private information, and the data collector has to satisfy additional requirements on how they store and process it. In particular, it requires verifiable parental consent. That clause in the Ts and Cs renders this null and void: "I am 12 and you have misused my data", "Sorry, you shouldn't be here."
The same clause is in the Ts and Cs for iTunes (hands up the people who are worrying about the 13 cut-off for FB but not iTunes) and for almost all other US websites that collect personal data. There are exceptions, such as Google, who don't collect verifiable parental consent and don't (so far as I can see) exclude children from signing up for account. Their argument would be that they are only collecting an email address; the counter-argument would be that it's now widely accepted, even by Google, that a search history is private information, and it's certainly sensitive private information under the DPA.
Whatever, claiming this is "the law" is over-stating it, to put it mildly. It's a US provision that has the effect of nullifying a US law enactable only by US residents for accounts signed up to in the US with (for practical purposes) US operators. Teaching your children to respect the law is one thing; teaching them to jump at shadows is quite another.