What a lovely problem/challenge to have!
I guess the first thing to do is to gear up some expectations for yourselves beforehand. Basically, it’s unlikely that there will be absolute silence as you make your vows: the key is to get a generic blur of noise rather than something really distracting. Even then, if a kid does say something loud and inappropriate, then the comedy will give everyone something to remember your wedding by, and people will smile at the memory.
In terms of including them in the service
This isn’t a theological question so much as a practical one. Kids are good at carrying things, and generally like doing so, so having them carry flowers or even the ring can be lovely. If one of the older ones is up to doing a reading, that would be very special. But there’s no ‘set’ role for them.
In general
One suggestion is that you allot one or more adults (who the children know) to be in charge of them during the service. Those adults (who shouldn’t be you or your fiancee and probably not your parents) will know that their job is to keep their allotted children entertained/ in a straight line in a bridal procession/ whatever. Sometimes a brother-in-law or sister-in-law comes up trumps for this kind of task.
