Let me start oiff by saying, we have a cat. We have children. A sandbox has a much different purpose to a child than it does to a cat, if you know what I mean. So when we decided to buy a sandbox for our daughter, we decided we needed one with a cover. I immediately thought of the sandboxes I had used at the daycare I had once worked at, the LIttle Tikes Classic Turtle Sandbox.
Now, I know you've seen these- they're those big, green plastic turtles you see at the kids' toy stores. That's it. But they're great! The lid pops on and off so easily, my 5 year old can do it, and there's no worry about cats (or skunks, or anything else) getting into it at night to "powder their nose" since the lid is held in place by two small "posts" on either side. (Hence why I added the hygenic rating.)
This sandbox is also unbelievably durable, as we've seen it jumped on, samcked with baseball bats, bumped into with the lawn mower, and much more, without any damage (maybe a scratch or two- hey, it's plastic!) We even leave it out over the winter, and we live in Maine, so that's saying something!
So, if you have children who want a sandbox but you're worried about animals getting into it, invest in one of these!