Here is what you can do when your child's playdough (whether homemade or not) dries up to refresh it so that s/he can use it as new again the next time.
This is only one of the methods that you can use:
Pour some water in its container (go over the dry playdough level). Close the lid. Rest it for a while (could be over night). Pour the water out. Start kneading the dough until it gets its original form (initially, it will be sticky but soon after, it will form its usual consistency).
Have a refreshing day!