Just a warning about using corn, I made a bunch of corn freezer/hot packs a few years ago. The extras (they weren't being frozen/microwaved periodically) got infested with something, I think pantry moths. It was disgusting, and I had to throw all of them away, and I did bake the corn first. I didn't have any trouble with actual beans in our bean bags. Um, not that I know of. I haven't cut one open.
By the way, I got the impression that the op meant bean bag chairs, not bean bag toys. I can't imagine purchasing enough corn or beans to fill a bean bag chair! Not to mention it would weigh a ton, and you'd still have the pest problem. How about recycling stuff that you already own? Packing peanuts, shopping bags, old pillows, etc. If you have a source for a grease fleece, and don't mind the dirty work of cleaning it, that would make a good filling. I mention that because I have 1 1/2 feed bags full of uncleaned grease fleece in my shed that need a big project:)
small bean bags = rice, beans, wheat berries, buckwheat hulls
large bean bag chairs = packing peanuts, cornstarch packing peanuts (more smooshy, so might not last as long), clean plastic bags, bubble wrap, polyester fiber fill, etc. I think real beans would be really, really heavy like 75lbs heavy.
I did mean a chair. I calculated for lentils, and it would be conservatively over 100lbs. Yikes. I've settled on buckwheat. It seems to resist pests and moisture and generally be a good weight compromise and has much the same feel as a traditional bean bag chair. Each chair will be about 45lbs, which I can move around, but the kids can't. I think this is actually good for minimizing the risk they will suffocate each other. The price is up there compared to a regular bean bag chair, but very good for a natural filling and cover one. Each bag 45lb bag of buckwheat was $75. I found a supplier who has these big bags for pillow and mattress makers in Seattle, which was great. I also discovered in my search that buckwheat hulls are sometimes sold as mulch. http://thebirkettmills.com/shop/mul-tex-buckwheat-hulls If you live somewhere it is carried so you don't have to pay for shipping it could be a good option. No one around here has it anymore, sadly.
If you have to order, I think this is the best price, though I didn't look into shipping costs: http://www.buckwheathull.com/
I'm typing this all up as it seems to be a pretty common question but no one ever updates with what they did. Hopefully now future searchers will get good ideas.
I'm now seriously considering making a mattress out of buckwheat hulls (more densly packed) for when the girls are ready for their own bed.
I made bean bags for the kids in the family this past christmas and used the Trader Joe's Organic Popping Corn kernels! I was going to use beans or split peas but I heard if they get wet then they sprout inside the bag. :) and with little kids drooling as much as they do I didn't want to take the risk. I was worried that Popcorn Kernels would have sharp edges that would poke out of the bag but NONE. They are super smooth, the perfect weight, and make the perfect bean bag sound - Good Luck!
See my thread on making a bean bag chair using any craft that can make a rectangle.
Hope this helps