You can certainly make homemade, but I bought a brownie mix to find out how many brownie bites one could make. The mix I bought was for fudge brownies and makes a 9"x13" pan full. I added 1/2 cup oil, 1/2 cup water, and one egg. I sprayed the mini-muffin pans with cooking spray and put a heaping teaspoonful in each. They bake in about 12-14 minutes. I happen to have several mini-muffin pans, so it didn't take very long. The mix made five (5) dozen brownie bites! If you don't have mini-muffin pans, put a heaping tablespoonful in regular muffin pans. The key is to make individual portions, not a pan full. If you make brownies in a pan, you are more likely to cut them in larger portions. Brownies cut in squares crumble more in going from the pan all the way to the lunch room via a paper sack or other lunch vehicle.
After they cool, bag them in a large zip-loc and stash them in the freezer. For each lunch, two or three brownie bites will seem like a lot to a child!