Have you ever done this with your kids? My Mum used to do this for me and my siblings when we were young and we loved it. I just remembered it the other day when I was cutting the tops off carrots whilst making dinner. All you do is take the top, put it in a little bowl (or on a saucer or whatever) with some cotton wool and keep the cotton wool moist. It doesn't take long for the carrot to sprout. Every morning at breakfast my two ask to check on its progress. Cheap, easy fun.