Years back when we first started I also was looking for something to help my son learn his line and similar things. I picked up an inexpensive FRS radio and earbud. I think they can be found at any kmart, walmart, etc for cheap. I doubt if you’ll spend $100 for 2 radios and the driver earbud.
I was warned by everyone that they were not legal to use during a race but we only intended to use for practice so I wasn’t concerned.
The FRS radios worked great, accomplished what I wanted and didn’t cost me much at all.