G'day Spiro
Mate, I had my son fishing in enclosed waters in the tinnie from the ripe old age of 3 1/2.
I virtually started him on the boat as soon as he was old enough to fit into a lifejacket. Safety is of course the first consideration. They only need to catch a small #fish or two to stop the boredom, but be prepared to take them back to shore if need be. Start them early and they will want to come out with you all the time, a great way to stay mates with your kids!
# # # # # # # # # Regards, Tony
PS By the time he was 6 he had already caught most estuary species, and many offshore species including trout, mackeral, etc etc.