With children that young, I'm assuming Fantasyland would be a priority. If you aren't able to do a MM, be at the gates prior to opening. Go directly to Peter Pan in Fantasyland......it is by far the most popular ride in this land. Dumbo, Alice, Mr Toad are the others that can stay quite busy all day. Fantasyland is probably the busiest area in the park (partly due to the amt of rides in a small area) but the crowds congregate there....especially if they have smaller children. So the earlier you can do this area the best.