My kids are pretty sensitive to chemicals and irritants, so I found this recipe years ago, and have been making it since before dd1 was born in 2011. It's great.
Supplies: 1 cup of flour; 1/2 cup of salt; 1 cup cool water; 2 Tbs oil; 2 Tbs cream of tartar; Wax paper
Procedure: Stir together flour, cream of tartar, salt and oil. Slowly mix in the water. Cook over medium heat stirring frequently until dough becomes stiff. Spread onto wax paper and let cool. Kneed dough with your hands until it reaches the desired consistency.
Note: You can add a few drops of color after you stir in the water (over the heat) to get colored dough. Get creative...you can mix 1/4 cup fruit juices with 2 cups of water, or you can add the juice from canned beets (straight instead of water). For browns you can use tea, for yellow you can mix turmeric with a little bit of water to make a paste. Once your dough is prepared, you can kneed in the colors.
I also have the recipe for finger paints and crayons (using bath soap).
If they eat it, give them plenty of water as salt dehyrates. However I think after they taste the amount of salt, they'll not do it again.
DS1 did it once, cried because the taste was horrible, I called his GP and was told to give him plenty of water but because what he had eaten wasn't toxic nor enough for an overdose, he should be fine.
In the unlikely event that your child does manage to swallow a large amount of playdough that contains salt, you should get your child to drink plenty of water and seek emergency medical attention. Consult NHS for salt doses and compare to how much you've used in the recipe and how much they've eaten.