There are heaps of recipes for play-dough. I find this one super quick and easy, because unlike some it does not require cooking or cream of tartar.
I actually like to halve the recipe below to make 2 separate colours, which then make a nice marbled play-dough once they are accidentally combined. Or, I make two lots of the full amount, send most of it off to kohanga with Sabina and leave a handful of each colour at home for her to play with.
In one bowl combine 2 cups of flour, one cup of salt, 2 TSP of cooking oil, and 1 cup of water (containing 1 tsp of food colouring). Mix with a wooden spoon, then tip out onto a floured table and knead well, adding extra flour if you need to. It will keep about a week if returned to an airtight container, but you will need to add a bit of extra flour to it every day.