Maybe I'm overly jaded from spending a bit too much time in close contact with politicians and lobbyists lately but I absolutely don't see this as much more than a savvy PR move in advance of a telegraphed regulatory threat.

UA doesn't guarantee that they'll make seats available they're just saying "if there are Y- seats available together we won't charge you the premium seat fee if you're traveling with someone under age, if not we'll let you change to another flight at no charge" (which for a lot of routes may be a bit illusory -- if you're traveling to Uncle Joe's for a weekend visit and the next flight with seats together is in 3 days (because there may only be a handful of options per day) are you really going to go with that option or are you going to tell the kiddo to suck it up for an hour?)