I thought there was a court case a few years back where a burgular doing a B+E fell down a flight of stairs because of a broken railing and sued the owners for negligence. Obviously it's ridiculous but in sue happy America anything is possible.
You have to consider the mechanics too. Those crossover machines weigh at least 300 pounds - the father would have to be doing some pretty ridiculous crap to have it actually fall over, never mind not noticing the machine was about to tilt over towards his kid.
He will have to live every day knowing he probably did something stupid and killed his son. All the gym has to do is get one of their staff to say "yup, we told him he shouldn't have his son in there but he ignored us." Lawsuit over. Imagine if the father had been doing overhead dumbbell presses, dropped one and it hit the kid on the head and killed him. Who's liable then?