Perhaps a longer time period for the contest, then you run into the problem of people loosing the post. the countdown times add a little bit of viability to a post so that might be of use to alert people here is a contest. For myself, I like looking at the results, but even if I did want to enter the marshmallow challenge I would have had to go to the store and hope they had a bag of marshmallows and some spaghetti. (not products I typically keep, and I usually only shop once or twice a week).
Maybe point the post toward people with kids of school age, I know there are a lot of people with kids. I am rather surprised that people with kids did not get their kids involved with the marshmallow contest, it seemed like it would be a great thing for families, perhaps they just did not have the stuff handy.
Also reach out to some of the communities that might have a larger child population, like Powerhouse Creative or the creativecoin, or the Home education communities. the homesteaders community may also be of help when trying to find participants. Some of them are small communities, but they do have some pretty dedicated users and are always trying to find fun ways to educate their children.