Younger Cousins In Wedding??

I have younger cousins (ages 7 - 12) who I am really close with their parents who are my first cousins. My two little girl cousins have already told me that they will be junior bridesmaids in my wedding. Something, I feel awful about, b/c I do not want children in my bridal party. I will have to let them down (most likely just by not asking). I fear that their parents (my first cousins) will approach me about putting their kids in my wedding party. I need to know how to address this issue. (I have already considered having them be ushers, readers, etc at the ceremony). In addition, I have many younger cousins in addition to the two girls who I have to invite to the reception. The issue is, they are bad mannered and do not behave appropriately (run around, scream, etc). I want to enjoy my wedding and not have a bunch of kids running around. I also dont want my older cousins to have to leave the wedding early b/c of whiny children. Is there any nice/proper way to address their parents about their children`s behavior if they are attending the reception? Advice needed :)