I'd never allow my kids to act like that on a bus. But, if there were other empty seats, and no one was standing, they weren't kicking or touching anyone else, who cares? As far as the boy being on top of the girl, have you ever watched kids wrestle around and play? At that age, my boys would have played like that and I promise they'd never seen anything like what you are inferring. Thats just how kids play when they are wrestling around. The fact that your mind went there has nothing to do with the kids or the parents.
Now this is totally different. If there are others needing seats then yeah, it's def rude to allow your kids to take up that much space.
I just don't get the judging parents for stuff like this. It's one thing if its causing harm to others or being disrespectful to others or something. But just because you don't think it's proper? And to assume that if their kids had gotten hurt they'd go after Disney? Are some people like that, sure. But I'm a big fan of the phrase "play stupid games, win stupid prizes". If my kids get hurt doing something stupid, it's no ones fault but theirs or mine. And most people I know are the same.
Also, there are plenty of times where it seems like I'm not paying attention to my kids, but I am. I'm aware of what they are doing. Sometimes I'm making a choice to let them work something out on their own, or learn a lesson the hard way because listening to mom is just too easy. Sometimes parents just get tapped out, especially after a long day at Disney, and you look at your kids and say they aren't hurting themselves, or anyone else, it's not worth the fight.
This is my least favorite part of parenting. Knowing no matter what I do, or which tactic I choose to deal with a kid situation, if it's in public, someone is judging me. I'm either too nice, or too harsh, or let it go too long, or should have just let them play or whatever. It's exhausting.
Im guessing these were you kids?