I have both movies on DVD and VHS!
There's an intermission (as referenced in those cute tv ads) and lots of fun trivia stuff in-between so there's plenty of things to keep the kids from getting too restless (I think.) Besides that, both movies are so engrossing that I'm sure the time will fly by for the kids!
And no matter how many times you've seen the movie on DVD, trust me, it is SO worth it to see it again in the big screen. The 3-D is cool - it's not gimmicky or anything, just adds to the animation like it did in Up. This weekend is the last weekend it'll be in theaters though! :(