Lemonade stands are stupid, and any parent that let's their kids do it are setting them up for failure. Want to teach your kids about economics? Do what the kids do now. In Madison, IN they have a bunch of crafts festivals and stuff all the time, and a lot of people will sell a can of Coke for 75 cents.
Go to Kroger's buy 5 24 packs (roughly totals out to about $22) and sell them at 50 cents to undercut the other person. In total the kid makes about $38 profit. Was it legal? I dunno but it was my g/fs "sweet" "innocent looking" 10 year old sister doing it, so no one ever questioned it during the hustle and bustle of the festival.