I built this sandbox for my girls last year, and quickly learned why my parents never let me have one ... what a mess! It didn’t help that what the material yard sold me as play sand, was really super fine masonry sand. It’s barely gotten used this year. My wife noticed that we could not keep the girls from digging into our pea gravel, and came up with a solution. “Why not take out the sand, and fill it with gravel?”
Why not indeed? And so today I did just that. The girls are happy.