Took our 3 year old for the second time over the weekend (coincidentally to see UP).
He sits there quiet as a mouse, litterally never seen him as quiet as when he is in the cinema, much easier than his six year old sister.
I wouldn't hesitate, if the little one doesn't behave you can always leave, and a little bit of noise is acceptable (it's a kids movie), you have to expect kids to be there.
BTW He is exactly same, will sit through a film and loves Cars, and generally all things Pixar 😛