Of course, radio kits are doable, and you can start with them, moreover, your kid would get a practical idea what is it all about: his own hand-made AM-FM radio. However, having said that, the any radio project should be accomplished under your supervision. I would recommend 8-9-10 years to start with, but everything depends on the skills and abilities of your kid. Let him watch and help you at the start and you will see how it goes, if and when he is ready to do a radio project without any supervision.