Children know that you always dress up when you go to something important. Whenever you have important visitors to your home you dress up. When you go to your boss' house for dinner, you dress up. When you go to the House of God, you wear shorts and a tee-shirt. Unless your children are incredibly stupid, they are going to attach the appropriate level of importance to God.
Church is a special place.
Bathroom break for everyone before you leave for church. Always get to Mass a little early. Try to sit in the front pew. From the front pew, children of all sizes can see what is going on instead of the back of some ten-foot giant. Always, always, always genuflect and remind each child as they enter the pew to genuflect to the tabernacle.