I would strongly suggest identifying a person to meet them at the altar and then take them to the back of the church to be entertained. I'm not sure what type of church service this will be, but even a Catholic "ceremony" without the Mass is nearly an hour! Nothing will entertain a young child for that long.
Have a box of quiet things at the back of the church where their babysitter can take them until just before the ceremony ends, at which time they can walk back down the aisle with you!
Ideas:
- Crayons and paper
- Color Wonder (no mess)
- small puzzles
- books that the sitter can ready quietly to them
- snacks (non-messy)
- water in a straw cup
- small cars or action figures
Keep in mind that when a bride invites children to be part of the wedding, she should be prepared for some noise. Kids aren't silent and they are messy. If she is the type who wants them to be "seen and not heard", then have an escape plan ready to go!
When my SIL got married, my niece (3 at the time and the bride's daughter) was in the wedding. She walked in and then proceeded to crawl on the altar. Her grandmother (father's mother, not the groom's mother) was on-hand to take her outside. She played on the swings at the church until someone texted her to bring the little one back inside because it was almost over. They had an escape plan because at that age... you're going to need it!