The tongue weight of the trailer needs to be less than 200#, but a small boat should be fine. I find that my 15' boat is pretty easy to launch by myself, so I get out lots. A 12' boat should be even easier. Thuggin and the other replies have some good advice on trailers. Remember that a small boat can just be disconnected from your Kia and moved around by hand, no problem.
Car racks are gonna be a PITA for launching an aluminum, glass, or wood boat. Trailers are easy to master after a few outings, and a boat like a 12-14 footer will provide plenty of options for you and your boy. My boat has been on bays, the Columbia, the Snake, the willy, and lakes.
Good luck,