Slow enough that he would survive a clumsy slam into a brick wall. Maybe 10 mph with 80-100lb load? Ride time of 1-3 hours would be great!
If you could find a wagon with a steel frame and stronger wheels then that would be a much better starting point. Or you might want to consider taking an easier route to providing something with electric power to your boy and purchasing a new or used Razor Ground Force or Dune Buggy electric go kart.
Nonetheless if you still want to add an electric motor to that wagon then we have all of the electrical parts that are needed for the project. We do not have any sprockets that would bolt directly onto those plastic wheels though, or any axles and wheels that could replace the existing wheels, so those parts would need to be sourced before ordering the electrical parts of the project to make sure that everything is available that is needed for the project.
Please let us know if you have any questions.
My boy has made it clear that if I am any kind of a father, this wagon will become electrified with a GO pedal, and preferably a brake, and ideally the motor/battery stowed in the compartment. He can steer with the handle, though I'm open to improvements on that too. I'm not very handy, could you recommend any certain approach and parts?