When my son wanted me to buy him a toy robot I asked him if he would like to build one with me instead, he was so interested so I told him that it will be a car robot with two motorized wheels controlled from the phone over bluetooth and he needs to sketch the design then build it using Lego blocks.
While he was busy with the sketches I started looking for parts, so I went to eBay and found these wheels
They come in two speeds, I ordered the slower one since it has more torque. These wheel will need something to control them since the Arduino micro-controller is not strong enough to drive them with enough current so wanted to build an H-bridge using transistors (which I have already) but I found a module based on the L298 chip which is very cheap and has a voltage regulator too, awesome.
I got them all from eBay with the cheapest price I can get so some took about a month to arrive and I wasn’t in a hurry:
2x Car Robot Wheels: $6.00
L298N Dual H Bridge: $3.00
HC-06 Bluetooth Module: $4.00
Arduino Pro Mini: $2.60
40pin Dupont Wires: $1.50
Total = about $17
You will also need an Android Phone or tablet, any with Android 3.0 and up should do. Lego blocks or anything to use for the car body.
Here is a video we made showing how to use the app and explaining the car we built and its main parts: