We ran into this problem when we were going to invite my fiance's small church to our wedding. The church has members with children that are very disruptive in a normal service (with a congregation that size... it is probably hard not to disrupt!), and we feared what would happen during our vows.

Here is how we handled our invite to the congregation:

Groom and Bride
Invite the ChurchName Family
To share as witnesses with their family and friends in their vows of marriage

Saturday April 11th at 2 p.m. at Church Name
Cake and punch reception to follow

In respect for the reverence of the occasion, we ask that adults only attend the ceremony.

I will toss in something... My brother was married 2 years ago, My sister-in-laws father spent over $200,000 on the wedding. There was hundreds of people there. The entire wedding was shot on three different cameras from different angles, then put together for an awesome video. I was visiting the other day and we watched the tape... the best parts were watching the kids... their looks, actions, etc... The reason for me saying this, I noticed a few suggestions of an "cut off" age. Well in watching this tape it is obvious that there are 4 year old kids with more respect and longer attention spans then some 14 year olds. I don't think a cut off age would help. If I were you with your situation I would send out invitations stating " My fiancee and I are throwing an adult party at which we will be married, Family BBQ and childrens party to follow on Sunday afternoon"
I would not want to be in your shoes, I do understand, Just Have Fun!!
Mike