$64,000 question horseshoes!
I saw one at a children's birthday party and one of the mums dished the sweets out. I googled the company afterwards as I was interested in this novel idea and noticed you could hiure an attendent too. But how you estimate the amount of sweets in an adult environment I've no idea!
Although this one suggests 1/4 of sweets per person. Have a look at their packages.
http://www.victoriansweetcartcompany.co.uk/sweet-carts/
Good luck with it horseshoes and let us know if you need tasters!