I use this one:
2 cups plain flour (240g)
4 tablespoons cream of tartar (52g)
2 tablespoons cooking oil (30ml/30g)
1 cup salt (320g)
2 cups boiling water and paint/food colouring* (500ml)
Few drops of flavouring/scent or some kitchen spices
*I prefer to use paint to colour the dough – it seems to give a more vibrant colour using less so works out cheaper. I use standard Early Learning Centre children’s ready mixed poster paint – 2 tablespoons should give a lovely vibrant colour. Bear in mind that it may stain clothes if you don’t use washable paint. If you want the dough to be more edible, use food colouring (but bear in mind that there’s a cup of salt in it so you really don’t want anybody eating it!).
Combine the dry ingredients (flour, cream of tartar, salt plus any spices for fragrance) in a mixing bowl then add the oil (cooking oil plus any fragrance oils).
Put your colouring into a measuring cup/jug, I find 2 around 2 tablespoons of paint gives a good colour, then top up the cup with boiling water and pour into the bowl of dry ingredients. Add another cup of boiling water to the bowl.
Mix thoroughly. If it doesn’t look like it’s working, keep mixing. As soon as you have a dough that’s coming together you can turn it out and knead it as this is quicker than mixing with a spoon, but watch your hands as it will be hot. Knead until you have a nice even and slightly stretchy dough.
Leave it to cool to a safe temperature then give it to the kids.
I didn't bother with the cream of tartar in the past but I've tried pretty much every variation and this recipe seems to be the best.