If neighbourhood cats are using your child's sandbox as a kitty litter box you will want to make sure it is covered to keep your kids safe from bacteria. In most cases it does not matter that the sand gets wet when it rains so covering it with a mesh tarp is normally the preferred option. This lets the rain seep through while at the same time keeping animals out.
You might even consider having the cover double as a shade tarp when children are playing in the sandbox. To do this you could build some sort of frame above the sandbox that you could hang a 90% block mesh tarp on when the kids are playing.
If you want to keep the sand dry, you should go with a solid vinyl tarp. In this case you will need a strategy to keep water from pooling inside the cover. If you are just covering a sand hill, this should not be a problem but if there is an actual sand box with sides like the one shown below, covering this with a solid tarp will cause rainwater to collect on top of the vinyl. To prevent this you might want to have a grommet hole added to the centre of the tarp so that water can drain. Otherwise, this pooled water can become stagnant and full of bacterial organisms.
Depending on the wind conditions where the sandbox is located and the type of material you use, you may or may not need special fasteners to keep the cover from blowing off. To be safe you might want to add a grommet in each corner so you can secure it with tent pegs. If you have an actual box, you can also add snaps that can fasten into the sides of the sandbox. Another option is to add pockets on either side of the tarp where you could insert chains to help weigh it down.
I recently learned that sandboxes have come a long way since I was a kid. Someone ordered a cover for an "Augmented Reality Sandbox". This piqued my interest and so I did a little research and found a Youtube video which describes these. It is actually pretty cool so if you are interested in top maps and elevation lines be sure to check out this Augmented Reality Sandbox video. You may also be interested in our play area covers.