I got my son a Futaba 3PV when he was about 4 and its worked out really well. He's 7 now, back then he had a couple of Tamiyas and now races 10th buggies and onroad and the 3PV is plenty good enough for that. The main thing for me is that its quite a small radio so he finds it a lot easier than my Sanwa MT4S.
Rx aren't the cheapest but they always pop up secondhand, and there are clone ones available now for about half the price of Futaba ones. Banggood have some for about US25 which work well so far
I don't see me replacing it anytime soon, it will probably break from being dropped etc before there is any need to replace it, it works really well.