Kids in Ceremony

I think I have posted this before, but am still wondering...I have three neices (10, 6 and 2) and a nephew (7) who are all very special to me. They are also INCREDIBLY well behaved. They are coming to the wedding because of our closeness and the fact that my sister can control them no problem. We are also inviting my FH both nephews (3 and 5), but my FSIL asked if her MOM could come to the wedding (who I dont' know) to watch the kids. I said no - we don't have the room, and the money to add people in. She said she didn't want to take care of them, but 'oh well, the grandmothers will be there and they can deal with them' I am hoping for the best, but in the meantime, I want to incoporate my sisters three girls into the ceremony as 'flower girl trio', and my nephew to walk my mom down the aisle. Is that too much with the kids? Honestly - PLEASE let me know. And what about the other two? Do I have to include them if I think they won't behave? Should I tell my FMIL that I am concerned? Sorry so long - any advice would be helpful.

I have our niece as our flower girl (3) and 2 nephews as RB (7, 4). I have seen some weddings were there are only children and adult BM and adult MOH. I think having the kids in the wedding is wonderful. BUT I would never leave out the other children. It would be hurtful, in my opinion. My friend's brother got married a few years ago and her SIL only asked her niece to be a FG, not my friend's daughter. She was very hurt and felt it was not right to her daughter. I agree.
Another one of my friends had her daughter (2 1/2) in a wedding. Her mother was invited to the ceremony and cocktail hour and then took the little one home with her so that at the parents could have a night out and not worry. Why not allow your FSIL to do that? It didn't cost my friend anything to have someone at the cocktail hour because no one at the hall was doing a guest count.
I think having the 3 girls and 3 boys would look sooo cute. And honestly if that day one of the kids is afraid to walk down the aisle, it will not matter. They will still look cute in the dresses and suits.