Thanks for the suggestions :)
Bubbles! Thanks, you've reminded me, we did had a giant bubbles set in the garden the other year and they went down really well. The bubble mix ran out quickly though, I'll get extra.
Chalk's a great idea, thanks. Blowy paper horns?! You're brave 
I think I'll do biscuits to decorate too, that always keeps them occupied for a bit!
I think DD would like some organised games. She likes duck duck goose and parachute games at sports-centre parties but they need loads of people (and a parachute obvs!). I'm not keen on games where kids get eliminated one by one as is just seems unnecessarily competitive.
We usually do musical statues but instead of eliminating kids who move, we call out a shape they need to be a statue of when the music stops ("a dog" "the happiest person in the world" "a square" or whatever) and the best one each round gets a prize. It's more of a laugh and ends up in fewer tears IME!
But, other than that I'm having a bit of a brain blank!
What else can you do with 7-9 year olds that doesn't take many people? They're growing out of the really young games but they're still up for a prize IME.
Can you do pass the parcel with just 5 DC? I guess you can? It's not the most exciting game, though, is it?!