Traxxas buggy la, around HK$2000 RTR, easy to buy spare parts and cheap, and can set ESC half speed for your boys, and full speed for yourself. Most of all, Traxxas plastic are very durable, and electronic are all weaterproof.
If you want something for kids buy short course la. I got a AE SC10 4x4 for my 6 year old son. And that thing is so robust the only thing he break after one month is bend one front drive shaft. Ask the ching's at MX club and you will know what kind of hell my son put the truck through.