The airline has no idea who is supposta be sitting in any given seat in F since the FAs don't check ID... And they're also used to people swapping for logistical reasons.
On both UA and AA, I have been elite for years but my wife isn't. I still apply for upgrades, etc., and my wife and I just trade off on who gets to sit in F for whatever particular leg. Never been questioned... and in fact, on one occasion, I've been given a beer from F as a "you're a smart guy" consolation prize.
As long as there is adult supervision of the kids, I doubt AS will care and/or notice.