whitelace gave excellent advice.
Another thing I will add is if you need to take the kids out, don't let them run around in the foyer. They will think it is play time and will figure out soon enough that it is more fun to run around there than to have to sit in the chapel.
It is best to sit them on your lap in the chair or the couch or even in an empty room (or on your lap on the floor) and have them sit quietly and reverently. It is still sacrament meeting time, time for reverence and they are expected to show that respect and quiet.
It may be tough for awhile. They might fuss or kick etc. Especially if the other little kids are running around, the foyer is rowdy and other moms or dads are chatting with other parents. We just tell the other kids/friends who come up to chat or play, ".Jake is having quiet time right now until he is ready to go back into the chapel." Even if Jake is fussing etc. I can walk him over to the pictures of Christ and quietly whisper to him about them until he calms down and is ready to sit quietly in my lap awhile before going back in.
You are the parent, be in control of the situation. Lovingly yet firmly. Let them know, sacrament meeting isn't a time to play. You might think..."But they are so young, it's such a long time..." Find what works for you and your family.