Birthday party ideas for bigger kids?

jenhum

I was just planning on Maddie's birthday being like a normal family gathering, where the kids all just kind of hang out and play- they are almost all well behaved and only 3 "little" kids, ages 4, 8 and 9. But my mom found out her cousin will be in town and invited her and her kids (which is fine, I really like them and we never get to see them), so that's 3 more, ages 3, 5 and 7 (I think, it's been a while since we've seen them).

So should I have something for them to "do?" Or just let them play? If I do plan something it needs to be very cheap (or free) and not disastrously messy.