The simples and cheapest modelling medium for this is salt dough. Mix ordinary flour (eg tesco vlaue self-raising) with lots of salt - to make it taste vile so kids won't put in mouths - if you want to make it viler add some nail-bite fuid (honest!) - then mix in enough water andknead it with your hands to make a pliable modelling medium. You can add colours such as powder paint to the body of the dough if you want bu it is not necessary.Make shapes, let them harden a bit then long slow cook in the oven to really harden them off. Then you can paint, stick on sequins etc
Try this link:
http://www.ancientnile.co.uk/saltdough.php