outsider:
One problem is two of your motors are wired in parallel to 1 controller output, so there is no way to individually vary their speeds, if you apply the same voltage to both motors, one might run 1000 RPM and the other 990, you would need 2 PERFECTLY matched motors with the SAME load to run the same speed for the same voltage, I doubt you have that with a low cost toy robot.
If both of those wheels are in contact with the ground, it will all sort itself out. It looks like a competent kit with appropriate motors for the task.