adriansmom had some excellent ideas!
I also think it depends on the childs age. Some children would prefer to have a seperate party, again this depends on their age. While other would have no clue or mind.
Having one huge family party sound like a great idea and then having individual childrens parties for their classmates and friends.
Ask the children and see what their opinions are... then take it from there.
BTW, what are their ages?