For what its worth, I agree with Henry, as I seem to do most of the time. (I never meant to be a yes man, it just happened!)
I always thought that the almond roca in the sand box was part of growing up. In my experience, (I have a 7 year old,) your five year old isn't going to use the sand box for much longer anyway. I think even five years is stretching it. Be prepared for globs of sand in your washing machine, because jr. thought it would be great to fill up one of his socks with sand and tie a knot in the top of the sock (don't ask!) not to mention the sand all over the yard, etc. But don't let me discourage you! *grins*
I guess I see sandboxes as a monument to the effort to build a better mouse trap for a rat. (Lots of effort for the wrong application...)
Best of luck to you, but don't make it too complicated...