The Haunted Mansion is no more dangerous than Small World?!?!? There's no harm in lap sitting?!?!
That is EXACTLY the kind of ignorance that gets guests injured or killed. It may be a "kiddie ride", but those ride vehicles are big, heavy pieces of machinery that can easily maim or kill. Read old news reports about the kid who was mangled on Disneyland's Roger Rabbit ride to see what can happen if you fall out of a ride vehicle. (Or the boy who lost fingers on a PotC boat flume)
Lap sitting may seem safe--and for the most part, I'm sure it is-- but it also overrides the effectiveness of the lap bar.
Either the child is squeezed underneath it, leaving other passengers with a huge (unsafe) gap above their laps-- or the child is held above it, which means the child is at the mercy of the adult's arms and hands to hold them in.
You've also gotta figure the size of the seating bench and the small passenger compartment that closes around you when the lapbar pulls down. Seriously, there is a limit on how many sardines that can be packed into a can. Is it four? Is it ten? In the case of the HM, Disney has decided that its THREE sardines!
Sure, you may have done it before. But who are you to question Disney's safety policy? They built the ride, they've safety tested it, they know how many cubic feet are inside the vehicles, and they will be the ones you sue if your child falls out and crushes their skull underneath the castors of the omnimover track.
My question is why its such a big deal for a whole family to squeeze together for an eight-minute ride? If your kid is too scared, do a child-swap.