I have two niece's and one was a jr BM and the other was a FG. I agree with PP that you can have another in charge of programs and maybe have 2 or 3 FG. To make them al feel special I would recommend that they all get to wear a nice dress that fits in with your colors.
My 2 nieces/goddaughters (14 and 10) and nephew (12) were our ushers. We had a small bridal party and no FG/RBs. The girls picked their own dresses and each got a corsage (bout for him).
Flower girls (I've been to a wedding where there were 6-8 little kids who walked down with streamers and pinwheels) Pass out ceremony programs Pass out bubbles, fans, water, etc. Ring church bell at the end of the ceremony Greet/Usher guests at the ceremony or reception Do a reading at the ceremony (if they're older) Decorate the "get-away" car
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Things the girls could do:
Flower girls (I've been to a wedding where there were 6-8 little kids who walked down with streamers and pinwheels)
Pass out ceremony programs
Pass out bubbles, fans, water, etc.
Ring church bell at the end of the ceremony
Greet/Usher guests at the ceremony or reception
Do a reading at the ceremony (if they're older)
Decorate the "get-away" car