Early this year, I discovered a product made in Sweden called Moon Sand. Moon Sand looks and feels like sand yet it can be to molded into any shape, like dough.
The baby bath tub I had purchased from Mothercare 6 years ago turned out to be the perfect size and height for containing the moon sand. As the sand doesn't stick or stain surfaces, cleaning up sand that fell out of the tub was an easy job. If there are more rowdy kids, I just place a large blanket under the tub to "collect" the sand that fall out of the tub.