Yes, its' true, with little kids it is important to have a spot in the yard where they are allowed to dig. But for me, any place that is diggable soon gets filled up with plants, I just can't help myself!
So this year I finally had a solution. Why not let them have the bare spot under a spruce tree? The only problem is that the roots make the ground so hard there that it is impossible to dig with adult shovels, let alone wimpy kid ones. So when I excavated for a new pathway elsewhere in the yard, I dumped the soil here. Now they have perfectly good, diggable soil in a place where nothing else would grow anyway. It ain't pretty, but it works for now!