Using a luggage trolley as a child mover doesn't strike me as sensible but maybe the perp had no common sense, hadn't seen the signs or had seen them and ignored them?
So just because it doesn't strike you as sensible a jumped up security guard should force him to stop it?
If the father was endangering other passengers or property then that would be reason for a security guard to stop it - but he wasn't. He was endangering
his son and, as such, the security guard should butt out of the guy's business. The airport operator is protected by the "don't do it" signs if the father was stupid enough to try and sue.
I used to love hitching a ride on the baggage trolleys and luggage belts as a kid - I can't see what security risk that these things give rise to.