Manners just aren't important to most people anymore. It just seems that nowadays (I can't believe I said that...I'm 26!) it is a common belief that "your kid" and his rights are more important than the rights of others...get your kid what s/he wants at all costs. It's really disgusting the way children are so overindulged and self centered. for some reason it's worse at Disneyworld! The rampant notion that b/c it's "disney" that children, no matter how ill mannered they may be, should be allowed to do anything they want, get the best seats, shove in front of others, scream and shout at all hours of the day or night (at the resorts), ruins the experience for so many others. I WEEP for our future.
To the OP--my 7 year old step daughter would have done EXACTLY the same thing your little one did, she'd have smooshed over and tried to accommodate the pushy aggressive child. It's such a fine line making sure that your kids aren't total pushovers but making them aware that they are not the center of the universe. Had I been you I'd have told the little girl that the spot was taken and to go and find her mother.
It used to be that if parents saw a child, any child, being rude, they would say something and correct the behavior or at least make it known to the parent. (I was raised in Texas and this is the way it was when I was little)The parent of the offending child would usually be shocked and correction would take place. But in the GIMME world we live in now, those children who have been taught appropriate manners and to be mindful of the world around them end up losing out. It takes a village people.