Start with the youngster in the ?? i would recommend to whereby you will see truck by using a brushed motor which will run about 30mph. The majority of the brands also have a step down switch near the controller to get the speed down to around 20mph so the younger child can get used to running vehicle.
Now I am going to describe an online controlled robot I am currently creating. I started with a plastic hobby box available from Radio Shack. I then mounted four geared DC motors on the bottom and installed the off road wheels. Everyone loves these geared DC magnetic motors. I got them from Tamiya Usa. Tamiya USA has a lot of really neat parts create a tool. All you need to do is Google robot parts actually find a lot of links to websites selling just what you are looking. If you are inexepensively you make use of the motors and wheels from a kids used remote controlled car. I have picked up broken remote controlled toys from second hand stores and hacked the actual motors and wheels.
By determining what scale you want, you can begin designing. If you make it too small you can have a tough time getting inside of details. Just one little side note for helpful keep in mind. Develop the measurements you to bring into play. Make your reference points at the tires. Using current kits for pieces, design difficulties that model car kits out like to make. You may like photos car, but see it with a distinct bumper or fender