I've got drawings of most of the components of this pump (and I'll try to finish them as time permits). Please be aware that this is a significantly complicated project, and it involves significant machining, plasma cutting, challenging welding, and considerable cost!
That said, both the kids and I are really happy with the result!
The pump is designed to be as safe as I can make it; all the moving parts are provided with hand clearance to minimize pinch points. The sliding part is the only remaining hazard and, on the advice of Chad Gehring, Director of Exhibit Design and Production at the Children's Museum of Houston, I used a rubber coupling to provide a soft push to keep hands out of the close clearance area. It is also designed to be as durable and abuse-resistant as I can manage. All the bearings and their shafts are self lubricating and replaceable. Sand can be poured over all the parts without damage. A screen is provided in the spout to keep out sticks and rocks. If the outlet is blocked, water comes out the top, flushing the linear guide at the same time. Both check valves are off the shelf brass cone checks.