I found this recipe in a magazine and it works a treat. Dylan has just started playing with playdough and he loves it.
1 cup plain flour
3/4 cup salt
2 tsp cream of tartar
1 Tbsp oil
1 cup cold water
Mix together and cook slowly in a saucepan, stirring continuously. When mixture comes away from the edges of pan it is ready. Take out and knead until smooth. I added the food colouring afterwards, but I guess you could do it before cooking.
Keep in a sealed container in the fridge.