Walmart currently has Cra-Z-Art Softee Dough Sets on sale for just $5. These sets usually retail for around $14, and similar ones are selling for over $7 on Amazon. While they're not available for shipping at Walmart, you can schedule a pickup at your local store.
Note that prices and availability may vary by region.
These sets are sure to keep the kids busy this summer, but if you need more ideas, you might want to check out the current Walmart toy clearance.