I look at it this way: If I am taking my family to Disney World, it is MY responsibility to see to their safety and comfort. If I think it would be unsafe for any of my family members to have to stand on a bus, then I see to it they don't. It is quite simple really. I would rather spend a few extra bucks on a cab or a rental car than leave the safety of my family up to the whim of a total stranger.
Sorry, but if you have small children and can't possibly stand on a full bus, then don't get on. Don't place yourself of your family members in what may possibly be a dangerous situation. Don't put convenience ahead of safety, and then expect others to hold your family's safety in high regard.
BTW, when I am able I do give up my seats for others that look like they could use it more. I just hate that so many people don't take it for a courtesy, but EXPECT it!