I built one similar to Good Knight's link as well but with some modifications. I made it slightly wider and I extended the rear of the mound with additional flat space for some extra comfort room when the pitcher was working from the windup. It also reduced "fall offs" when the pitcher was receiving a throw back from the catcher. I added rubber yoga mat material, gluing it to the plywood before putting the turf over top (a large strip from about a foot or so behind the pitching rubber almost all the way to the bottom where the front foot lands, not quite full width of the mound). It provided a bit of cushion and reduced wear and creasing of the turf and allowed a bit of give for the occasional kid that would get on the mound with cleats. Lastly, I screwed two grip handles at each end to make it a little easier to lift and carry. That said, my mods made it heavy. I was able to move it around the yard but I wouldn't say it was really portable. It would have helped had I notched some more of the wood out of the 2x's.
It is important to get the slope right and it will surprise you how long the mound needs to be to accomplish this.
I definitely agree with what adbono is saying about having the kid taking ownership but not sure how many would put it together well enough. At least make them part of the process at least for sure.