hmmmm I think you should have her as your flower girl, there are no rules stating a flower girl must throw petals. She doesn't have to throw anything at all, she can look cute just walking down the aisle in a cute dress or carrying a cute pail of flowers or throwing confetti.. anything the list is endless.... and then that way she can be by her mother through the entire ceremony and you won't have to worry about anyone else holding the child or worrying about her.
I understand its your wedding day etc.. but there are many ways to make it work... for everyone