I've been seeing a "clean mud" tutorial making it's rounds again but it involves shaving 3 bars of soap. I prefer less work and prep for a fun activity where the longevity is unknown ;)
"Clean" dough is SO simple - baking soda and water. Pour some baking soda onto a tray or in a bowl.
Then pour in water little by little
Mix until you get a consistency you like
If they start to lose interest add in a squirt bottle of vinegar for fizzy fun! The best part is that clean up is super easy! Have fun!