We were in this situation last Oct when my sister got married. I was the maid of honor and my husband was an usher. While my husband was doing his ushering, my aunt from out of town was willing to keep an eye on our kids (who were flower girl and ring bearer). I know my sister and her fiance were going to ask my husband to be a groomsman, but decided instead to ask him to be an usher for exactly this reason. I know your husband will be occupied, so are there any family members or friends attending the wedding who can have the kids sit with them during the ceremony? My hubbie ended up having to take the kids out almost immediately after their part in the ceremony.
It was a very long day for my kids and I thought they would be ok through it too, but we were literally going from 9 am and by the time the reception came in the evening the kids were besides themselves. We have a neighbor who was able to come to our house and babysit for the reception. My husband brought the kids home, put them to bed and the babysitter took over from there, and he returned to the party.
I'd recommend you bring someone to the ceremony with you who is not emotionally attached to the event and will be willing to take the kids out if needed. I personally thought it was too much to ask a family member to potentially miss out on the ceremony to watch my kids, and was grateful my sister and her finace asked my husband to be an usher instead of a groomsman. Then this person can take the kids back to the hotel and watch them if they get overtired too. I think you may find out of town family you will want to reconnect with and it's virtually impossible (for me, at least) to do that and keep two young children in your sights and occupied.
You didn't mention when the wedding is. Do you have time to build a babysitting relationship with someone in your neighborhood or through your church? You could ease this person into the kids lives between now and the wedding and by the time the wedding is here they will be used to each other. You may appreciate having a babysitter after the wedding too.
Before my sisters wedding, we never left the kids with anyone but family either, but started building this relationship with our neighbor and my husband and I love getting out of the house for an hour or so every week to play tennis.