Whoa....slow down here a sec.....flaps on a trainer to learn with is not a good idea....you're gonna have your hands full at first just learning to fly....trainers don't need flaps anyway as you can cut power on the down wind leg and still float on in to a great landing. After you learn to fly you're going to want something a bit more zippy anyway. Then you can always add flaps to learn and play with after that second airplane you're going to want.
To answer your question, I'll ask you one. If you were a qualified RC pilot now what would you like to fly? A big areobatic gasser? A detailed war bird? Or something a tad smalled, perhaps a 90 sized glow to fly around with. If the answer is yes to any of that...well they are all 4 channel at least, so learn on 4 channels with a glow engine. If electric is more to you're liking then learn on an electric powered airplane. The Sig LT-40 Kadet is an excellent trainer.
One of the hardest students I ever had was an old glider pilot...he never had throttle or ailerons....you're primary turning surface is ailerons, and throttle becomes altitude control especailly on approach....he had to un-learn what he was used to and re-learn 4 channels. I'm telling you this because you mentioned a glider...after you know how to fly have a ball but while you're learning stay with 4 channels.
I'm glad you said something about an instructor....he's who you should talk with as to what to buy to learn on.
Now this of course is just my opinion....the goal here is to learn to fly, have fun and make memories of Dad and me for your son....Good Luck