Play Grade or Serious RC car choice will somewhat determine the budget. Keep in mind a 6-year old will damage a play grade vehicle just by looking at it. And a more serious vehicle will not only cost more, will be more complex and will likely require parental supervision. Think this through carefully. "Speed" is NOT what a six year old needs in an RC car. If the car is powered by a lipo battery YOU need to become familiar with battery charging safety. You cannot leave a six year old in charge of a charging lipo battery because, quite frankly, they have the attention span of a gnat. It is nice you want to get him started early but this is where parents need to be parents and provide guidance. Do this together and then gradually ease him into his own equipment. I guarantee your first rc car will only be the first of many to come.