LONDON, June 9 -- The message from Washington to Britain this week was, in effect, that the country should think twice before deciding its own children spend too much time on American apps. It arrived not as a tweet or a phone call but as a formal submission to a British government consultation, lodged through the US Embassy in London, and its target was a policy that has not even been announced yet.

As Prime Minister Keir Starmer prepares to bar under-16s from social media, the White House used the consultation to argue against what it called broad social media bans for the roughly thirteen million British children the policy would cover. The Trump administration said it does not flatly oppose age checks, but prefers them aimed narrowly...