OK, Kiri - looking lively (I am in yet another hotel room, and anything is better than concentrating on my presentation for tomorrow, even though I must do that soon)
Musical Statues - TBH, at 6, I wouldn't bother with a prize for the winner unless you are 100% sure that is what they expect. 6yos of my acquaintance still prefer a sticker as you get out (also handy for telling which of the DC dancing are actually still in) and it always seems to get down to the last few uber-competitive DC, at which point I give it a couple of turns and then declare them all winners [coward]
Extraneous parents - um - not good at this one as most people generally still stay at parties here (very rural, massive school catchment area (independent school), not worth going home, and also the only time a lot of us see each other). Is the party at home (in which case I can see why you want rid)? How about some breezy wording on the invitation to the effect of "no need for parents to stay if you could just leave me your mobile number...." ?
If they insist on staying, either put them to work, or, if that is more trouble than it is worth, give them coffee and ignore them. Is 4 bear/child wranglers going to be enough to supervise bear stuffing, though? I have only been able to do stupidly ambitious craft stuff because I know a reasonable proportion of parents will muck in and help - are you going to be able to sew up bears and serve food and take DC to the loo and....? If not, you could phrase it as "no need to stay, but you are welcome to stay and help if you would like to" - that would get rid of most of them 