How about letting the older children in the group help out with playing games and things with the younger kids. They can be in charge of the music for the games you play - e.g. musical chairs, pass the parcel, dance and freeze. Maybe they can organise a treasure hunt too.
You could put out a table with paper, pens/pencils, glue and collage materials and they could decorate their own party hats.
I made a fishing game once for one of my dd's parties - i cut out paper fish in different colours and attached a paper clip to each of them. I made fishing rods out of bamboo stakes from bunnings, added string and put a small magnet on the end. They had to go fishing for small prizes. They had heaps of fun.

Good luck!