You can't really give kids lottery tickets, can you? How about adding in a minature of brandy and a cigarette lighter, just to make it really fun?
I like the lolly idea. Each child wins a lollipop, and the performer wins something else. If this were a full kids show party, I'd suggest a custard pie. And we know where it would end up.
Of course, what with food intolerances, hyperactivity and the old "never take sweets from strangers", thing, it might be as much hassle to try and give the kids sugary sweeties as giving them scratchcards.
How about giving them each a picture to colour in later (although I suppose eight year olds might have outgrown even that), and the performer has something like a fully coloured in Mona Lisa. Or maybe even one of those magic pictures that seems to colour itself in?
I was actually wondering if you could get away with your picture being a photo of an attractive lady (nothing too racy, of course), but I think it's best you steer clear of that entirely.
Or perhaps the kids get cute animals, cowboys, etc. to colour and you get this:
http://www.flickr.com/photos/jawbonerad ... 171460301/