We have recently started taking DD who has just turned 2 to the movies. We started off with a short movie (a 50min Dora movie) and the next week DH took DD and DS to see The Smurfs in 3D. She was great both times, and apart from not getting the concept of 3D glasses we didn't have a problem with her at all. She happily sat there eating popcorn with her brother who is nearly 4 and a vetran movie goer.

So I would say go for it. The worst that can happen is that they have to leave if DS becomes upset or restless. I would recommend a short-ish kids movie at the first session of the day so less likely to be tired and grumpy where you will get the family groups that don't mind a bit of chatter and running around.