That's workable. What would you hide?
I don't have an epipen so if it's sweets or such and the kid has a reaction to nuts (or similar) then we could have trouble, so would small toys (eg pull-back cars, bubble blowing things, plastic animals etc) be a better thing just in case? There MUST be some things for both genders. Toys should be of similar value to encourage swapping amongst the kids - if, for example, your guy finds a princess' tiara and Kendall finds a model bicycle, they could exchange them and not feel like they got a bad deal.
To make that even easier, once we know how many kids of each gender are going, we could wrap the 'treasures' in a gender-indicative colour and tell the kids that if they're a boy they need to look for blue treasure and pink for girls, that removes the entire gender issue.
It's all just ideas for now.