as you may well know, the best 'child' friendly mark would probably be a pier. it all depends on just how young he is and to what type of mark (pier, beach, rock) you are prepared to take him to.. regarding safety and ability
regarding the north coast, there are a few piers to choose from but being assured of catching fish is another story!..
portrush pier? - expect small dab, coley, pollack and a few other small things. maybe something decent who knows. its usually sheltered and can be busy, but it might keep him happy
good advice there from rushnaldo, a few rays should have him happy from rathmullan pier