Well there are a few major issues with your idea.
Motors are quite heavy to move. The initial amount of torque needed is significant (i.e from resting to moving), and therefore is out of the question A servo version of this (as I detailed) is far more sensible. Correct me if i'm wrong.
Another issue is, why the hell would you need two motors at the bottom of this?
Many robots only ever need two motors.........and that's it. One motor moves forward. One backward. The robot now turns. None of this 'pin' stuff.
If you were to do your design, maybe a servo at the top and one motor driving both wheels at the bottom. But then, as I said, you might aswell be sensible and do two wheels by themselves. word.
Seriusly, though, build ONE robot. That's all i'm really asking of you. It can be as rubbish as you want..........uncomplicated........whatever.
Actually FEEL the torque on a servo. FEEL the torque on a motor. APPLY different voltages to a motor. Does it move? Does it move faster given different voltages?
I urge you actually do this. Before you do anything else, ask any more questions etc etc.