If they are used to watching movies on TV (or even just TV programs that are shorter) at home, then they are also used to being able to move around, to talking and reacting out loud, to being able to stop and start the program if it's on DVD, etc. They have no real concept that in a theatre you cannot be popping up and down, going to the bathroom several times (and missing things, because the film won't stop for you), talking out loud, etc. I would for now check the movies you see in theatres carefully and ensure they are short (the latest Winnie the Pooh was just a few minutes over an hour long, for instance, but many animated movies these days are a full 90 minutes or more) and that they really are appropriate. A lot of movies are SO full of loud music, loud noise, constant action that I don't see how anyone can stand them for more than 15 minutes. The huge screen and especially the IMAX versions can totally overwhelm some kids and make them upset if not just restless.
Check out all movies beforehand. One mom posted on here about how shocked she was at the "violence" in "Puss In Boots" when she took her young daughter (whom I think was about 4), but she should have checked all the reviews and paid more attention; it's a movie we all loved, but clearly it's not appropriate for a kid that young in length or subject matter or amount of action. (My daughter's 10 and it's more for that age.) There are plenty of online review sites aimed at parents choosing films for kids to see.
You can remember going to movies at this age and I can too, but the movies back then were nowhere near as loud, action-packed, busy, in-your-face or halfway aimed at grownups as "kids" movies are today. I am not against them at all but I think kids may need to be older to really understand or appreciate a lot of today's kids' movies, and parents may need to forego many of the PG ones in favor of straight G movies -- which is more boring for parents but maybe better for little ones.
Also - be sure you are not getting there so early that the kids have to wait through the long advertisements and other junk they put on the screen before the actual film begins. We sat for a full 25 minutes of advertisements and film ads before seeing "Puss in Boots." Imagine your younger kids having to sit through all that BEFORE the movie even starts.