There is a material that I discovered some days ago, it's called Foamy (goma EVA in spanish). Its perfect to make crafts with children, it can be used in several ways; you can cut it, paint it, glue it... I thought it could be nice and easy to make stamps with it, here you have some examples but I'm sure that you can do it better! If you want more stamp tutorials here you have another one I made.
Some support; bottle top, small box, candy box, piece of wood, nail polish...