Modeling Clay also known as Plastilina is non-toxic and is mainly used by kids between 3-8 years old.  This clay remains malleable even when left for long periods in dry environments.  It can be a little bit hard when it's straight out of the package but becomes nice and soft when squeeze it and warm it with hands.  i tcan be re-used over and over again so it can help kids develop their concentration and build self-esteem through trial and error as it can't be made permanent.  Normal shelf life is about 2-3 years.