TJ is right the radio is not legal in competition – However, it is legal for every other on track practice session and is one of the most powerful training tools if used correctly and by someone that hopefully has some kart racing experience.
I see many dads coaching (and I use the term loosely) their kids and they’ve never even say in a racing vehicle of any kind. Needless to say, they’re the ones screaming at the kid over the radio the whole way round the track.
If I may suggest,
1) Buy these radios- http://www.cardosystems.com/motorcycles/scala-rider-g9x-powerset/1
2) get some tips from some with experience in coaching with a radio on how to speak to your kid over them.
3) Most important. Make it fun