While I'm not disputing your statement, I haven't seen this myself in the US, unless the parent is sitting in an exit row. (I have seen it outside the US, many times, mostly in countries where it is required by law.)

If the seats were assigned by the airline, are they moving other passengers to make it happen? The skeptic in me wonders. It seems to me that using this reason leaves the door open for insisting that they move others to make it happen when the seat assignments are not the passenger's choice but the airline's. (And when there are no empty seats available.) I have a wee bit of difficulty believing that any US carrier would voluntarily set a formal policy that would force them to put families ahead of status passengers in the seat re-assignment protocols, and to delay departures while they reseat all families in blocs.

FTR, I don't think it is a really great idea for kids to sit separately from their parents when they are on the same flight, and I don't split my seat assignment requests on purpose. Of course, I have no issues at all with sitting in tail of the plane, either. (OK, one issue: I prefer not to sit in a non-reclining row on an overnight flight.) However, as long as the airlines are not required to keep us together, I'm not going to have kittens when they don't.