Mix all ingredients in a pan. Cook for 5 minutes on medium heat, making sure to stir constantly until the mixture becomes stiff. Store in a plastic container in a cool place (fridge is good). This fun dough will last quite a while and is a joy for kids to play with and explore.
To seal your work, all you need is a water-based sealer of some sort. I use Flecto Varathane Elite Diamond Finish. It's non-toxic and fume-free. You can buy it to paint on or as a spray. The spray is easier to work with because you don't run the risk of brushing pieces off of the finished work ... but either one will seal the pieces so they are hard as rock. This even works on pottery clay that is meant to be fired in a kiln (you know, the stuff that normally doesn't last more than a couple of days if it isn't fired). I have students who still have clay busts from 5 years ago that are still strong as anything ... never fired, only sealed with Flecto. Its amazing stuff.
Another option is to make a glue and water mixture that you spray (use an old glass cleaner bottle) or brush carefully onto the piece. All you do is take some regular white glue and water it down so it is slightly liquidy and apply it to the playdough pieces. I'd still recommend the store-bought sealer but the glue will work in a pinch.
KinderArt® Recipe: Non-Edible