A Sandbox Cover is an essential design component of any sandbox you might consider for your children. The sandbox cover will keep out the neighborhood cats and prevent you, and your children, from unnecessary exposure to a parasite called Toxoplasma gondii that causes a very serious infection called; Toxoplasmosis. This is particularly important if you are pregnant.
A sandbox cover will also keep out leaves, twigs and general yard debris, even bird droppings (depending on the cover) and keep your sandbox environment nice and clean for the ones it was meant for.
Unfortunately, it wasn't until after we had our sandbox built that we realized how important having a sandbox cover was. It seemed like every cat in the neighborhood was using our kid's sandbox as a litter box. We quickly realized that trying to clean out the cat poo was not only a battle we couldn't win, it was also dangerous because of the Toxoplasmosis.
We also had a child that liked to eat sand and the thought of her eating anything other than sand was pretty disgusting! Eating sand was bad enough.
It was a this point we decided to make a cover for the sandbox and foil our furry, four-legged neighbors. We further determined that it should be easy to gain access to the sandbox quickly whenever we wanted. So I manufactured a cover/lid utilizing 2 pieces of 1/2" plywood that were 4' x 4' (the sandbox was 4' x 8'). They were essentially like 2 doors. The idea was to have a hinge on one side so the doors opened and rested against the fence that was right beside the sandbox.
I used a piano hinge (although now I'm not sure why as regular hinges would have worked just fine) and attached a handle to each of the doors to make it easier to grasp for opening. This worked quite well for a few years. The downside was the doors were too heavy for the kids to open when they were younger and eventually sand, and rust, made the piano hinges inoperable (they really aren't designed for outdoor use).
The next thing I tried was building a frame (again 2 - 4' x 4' pieces) using 2" x 4" lumber and good sturdy outdoor hinges (I learned my lesson from the piano hinges). Then I covered the center with hard, ridged, plastic lattice. It looked great.
The problem with the latticing was two-fold: It didn't keep out yard debris very well and one of the kids just had to walk on it to see if it would support their weight (I used to think that my kids were the only ones that did stuff like that). The lattice didn't support their weight and there went all my work.
Probably the easiest, and quickest way to cover your sandbox is with a manufactured soft covering. Some of these are available as a sandbox cover alone and some are available together with the sandbox. They usually have a type of draw-string around the outer edge to tighten the covering and they recommend a large beach ball be placed under the cover to give a pitch to the cover to direct rain off of the sandbox (so it doesn't pool on the cover over the sandbox).
Apparently, the water still pools in the cover and it's difficult to get the cover off without water getting into the sandbox. Some complained about this limitation of the soft cover, while others simply accepted it and commented that moist sand was better for playing in and molding.
The other negative comments regarding the soft covers, was that they seemed to fade quickly in the sun. From my experience, I feel that as long as the integrity of the cover is sound, and I could get a couple of years of service from it, it's more important to have the sandbox safe than to worry too much about the cover fading.
Because they are manufactured to a certain size, and recommended for their particular sandboxes, be sure that you take that into account before you begin building your sandbox. You will want your sandbox cover to fit when you have completed your project. If it were me, I would first decide what size I wanted to build my sandbox, then I would see what covers were available and then purchase one. Then I would build my sandbox using the cover as a template.
There is also the option of purchasing a relatively inexpensive tarp and grommet kit from the hardware store to make your own soft cover. You are welcome to try this, but I have found from my experience these grommet kits to not be so great.
Just in case you are wondering, I replaced the sand after I installed my sandbox cover, with fresh clean sand. I would strongly advise you to do the same if that is where you are in this process. The old sand can easily be dealt with by spreading it out over your lawn or working it into flower beds.