If you want your kids to grow something fast that they can eat too, try radishes. Seriously one of the fastest growing (and therefore rewarding things to try for kids). I let the kids plant some radishes in a little pot that has been sitting by my kitchen sink on February 19 and this is the picture I took yesterday, February 23.
This is the picture I took today, February 24 after only 1 day.
Notice how thickly crowded they are? That’s because the 6 year old and the 4 year old planted them for me, mixing two varieties of radishes. This pot has been doing amazingly well, especially considering that we are having this much growth and it is getting absolutely no direct sunlight. After we thin them out a little, they can grow and the kids will like putting them in their salads. From start to full maturity (according to the packages) was under 30 days. Kids are loving it.