Have you considered a pea gravel sandbox? It would have the same properties as corn as far as sandboxes go. Namely, you can't form it into castles, kids might swallow it, and it's more expensive than sand. However, it doesn't have issues like rot, water absorption, insect attraction, and bird attraction. However, if you want to go with corn, here's what I have on that:
The agritourism farm in my town has a giant corn box and it is spectacular. I asked them the following question:
I am thinking of building a corn box at home like the one you have
because it is incredibly awesome and sand is awful. Do you buy feed to
use or grow your own? Do you have to treat the kernels ahead of time
or replenish them during the year? How do you prevent rot or birds
just eating all of it? Is this completely not worth my time if I don't
have access to a large volume of cheap kernels?
They responded with:
We buy the corn from Southern States in large bags. Much better than sand for sure! We do clean the corn and switch it out throughout our season. If you can it helps to cover to keep birds away.
Their corn boxes look like they have 1x6 boards for the sides but I can't tell how thick their bottoms are. They also have it under a roofed area that is at least twice as long as the box in both directions. There's nothing to protect against birds, though, unless they put on some kind of removable cover after they close. Even then, though, it appears they have to deal with rot and contamination since they change the box throughout the season. It's a very popular attraction for the kids, though, so it's probably worth it for their business to do so.
If you want to build your own, I recommend:
- Making the box be as tight a fit as possible. It might be worth buying a plastic liner or a pre-fab box with a lid.
- Keep it covered when not in use with as tight-fitting a lid as possible. If you go with a plastic liner and it gets water inside, it'll be trapped and you'll definitely get rot.
- Consider sloping the entire bottom of the box to a single point, cut a hole in the side, and install a check valve to let water out if it does collect. The coarse nature of the corn - as opposed to sand - should allow the water to flow freely.
- Churn the corn regularly to check for evidence of rot or other issues and replenish with fresh corn. This is the part that might hurt your wallet and it might depend on how well the overall design is.
- Spray kid-friendly bug repellent around the outside.
- Mix in cinnamon to help keep the bugs out.
Of course, some of these are good ideas for regular sand boxes, too, since they're prone to bugs and moisture issues.