I recently taught my girlfriends cousins to fish. They are 3. Lets just say it was a dangerous experiance. Alot of enthusiasm, but also alot of hook flying around in the air.

Seems wat worked best for me was the 'let me cast it for you' Then either let the fish basically hook themselves on a bobber setup, or hook the fish myself and then hand it to robert saying "Here you go, i casted it for you' waiting a couple seconds and then exclaiming 'I think you might have one! Reel it in!"

That seemed to be the safest way to do it, although if you dont actually need him to cast it could be safer that way altogether, either way im sure it will be a fun day.