I was there for a month last August and will be there again this August, but south of you in Summerland Key. If you want to get kids on fish, one of the easiest methods is cut squid on a Carolina rig type setup with a barrel weight and circle hook. Being that your son is 12, you might want to start with a smaller shark so he can handle catching it. If you are fishing for mangrove snapper with cut squid, you will also catch some sharks.
If you want to target tarpon, cast net some live bait and do some freelining. Try to get off the beaten path and you will find more cooperative tarpon.
As for dolphin, I'm not sure I would take a 21' Grady White that is not my boat and I am not familiar with out to where the dolphin roam, especially with kids. If you do give it a try, pick a calm day. Get out to 400-600 feet and troll ballyhoo with and without skirts. If you find a group, have some cut bait ready so you can chum the water and keep them around.
That's the very short version. There are 1000s of articles on the net about how to catch tarpon, dolphin and sharks.