These plastic, all-in-one sandboxes with covers are admittedly smaller than most of the sandbox kits, but are great for younger children, and their use is not limited to use as a sandbox. They consist of 2 parts: the body and a lid/cover, which are constructed of highly durable commercial plastic.
The base is usually molded into an interesting shape, which children often become very attached to. I have read reports of children receiving their sandbox as a gift during the winter months and wanting to play with it in the house (without sand of course), and even wanting to sleep in them.
The base can hold water so these sandboxes have often doubled as a wading pool. And because the sandbox base is self-contained they can be placed on a patio, terrace, or deck. You wouldn't normally do this with the larger kits as most of them have no bottom. That also means that you can move them to another location if you want. Because grandparents may have sized down this makes these sandboxes attractive for them to purchase as gifts. Then their favorite little people have a play place when they come to visit.
Also, because the base is plastic it won't rot like a wooden sandbox, and will therefore last much longer and will be available to "pass on" to the next child.
These sandboxes have a cover, or lid. The lid will keep out the neighborhood cats and other unwanted yard debris. The lid is also important for these sandboxes to keep out the rain. With the kits the rain washes right through, but because these sandboxes have an enclosed bottom the lid is necessary to keep the sandbox from filling up with water.
A very nice feature of these covered sandboxes is that they are easy to clean. If the sand starts to get old, just get rid of it and clean the base out with soap and water. That also makes them easy to empty and store at the end of the summer season. Because they are smaller, they don't require huge quantities of sand, so you can put new sand in them every year at a limited expense.
Please find below a classic example of the all-in-one plastic sandbox: I hope you're not put-off by the commercial video, I found it to be useful for giving the scale of one of these sandboxes, with children actually in the sandbox playing.
Step2 Naturally Playful Sandbox