Give the kids a task/game they can do while to keep them busy while you're getting ready, and even at the ceremony.
When it's time for any formal portraits and the kids are not smiling (no matter how many silly jokes, tickle monsters, or animal sounds I make) it's okay to have someone they are comfortable with to stand right behind the photographer--like, right behind the photographer to get them to look, smile or even laugh.
Make sure that who ever is taking care of the kids knows their eating/napping schedule. Be sure to try and stay on schedule or at least have not-so-messy snacks on-hand for them-- unless of course, the ice cream truck passes by. I mean, #theyonlyliveonce
Just like adults, kids usually don't know what to do with their tiny little hands. So, let's give them something to do. Have them hold a small bouquet of flowers when their lined up with the bridesmaids, for example.
During the reception, have games and activities they will enjoy. Some ideas may include, a table of crayons, and other arts and craft activities. Or have an area where they can play with the other kids like a toss across game, or life-sized Jenga.
At the end of it all, let them have fun too (just make sure to designate a responsible adult with them).