Step 1: Materials, Tools & Time
You can get everything you need at Lowe's or Home Depot.
1 - 3/4" hardwood dowel
1- 4" x 1" Pine plank (I'm calling it '1x4' here)
5 - bags of #216-1/2 small screw eyes (eyelets)
1 - roll of braided nylon string (one color for ligaments. We used yellow)
1 - roll of braided nylon string (another color for tendons. We used pink)
1 - bag of 1/4" wide rubber bands (like the Post Office uses)
1 - cup hook (open eyelet)
1 - can of white spray paint
2 - 3/4" sheet rock screws
Various hand tools (you already have them if you are a maker! :)
We used a small drill press, 4" side grinder, jig saw (my Dad taught me this was called a "Saber saw") and a chop saw with a trim blade. These are time savers, but you could do it all with hand tools if you can spend the time.
Assuming you have the power tools, you should be able to easily build a hand in about 8-10 hours. That includes taking time to teach your young experimenter how to safely use tools.