Oh, sorry, I misunderstood. Here they are:
1) Get a buddy box.
2) Get a buddy cord.
3) Get an instructor to operate the radio while the kid holds the buddy box, which is hooked to the main radio via the buddy cord.

Sorry if I won't endorse someone trying to fly on their own. I personally think it's a waste of time, risky, and a silly way for most who attempt it to prove us right when they crash. Very few who try to learn on their ever succeed. Most give up after crashing their plane one or more times. I can't remember the number of posts I've seen from people expressing regret that they even tried it on their own. So, if the kid insists, tell him (or her) to lose the foolish pride and get help from someone who knows what the heck they are doing.