I am with Mary on this.
Cam cleats do not control the load. If the load gets away from them, the cam cleat is of no use.
What is traditionally used is some sort of Belaying Post, Dead Man or Snubbing post. That is, a post between you and the pulley and load which you wrap the rope around and the angle of wrap or number of wraps allows you to control the descending load with a light hold on the tail end. Trees come with these, and after fighting to tie off, and untie a load a few times, your kids will have invented this for themselves.