Hmmm...not recommended for a couple of reasons. First, if your code is wrong and you turn the motors on at 100% duty cycle they will carry excessive current and can be damaged over time. Second, 400mA is actually probably a lowball estimate for these types of motors especially when they stall. Third, these motors are going to generate a lot of "back emf" noise which is going to couple into your Arduino board and could actually damage it if you're driving directly from 5V.
Why not hook the motors up to the 9V battery (which is probably a bit underpowered...AA's have more current capacity) directly? (through the motor driver of course). You can then use PWM (e.g., analogWrite()) to set a 33% duty cycle which is comparable to an effective voltage of ~3V, just like in the original car. The 9V battery (or whatever) powers the Arduino too, and since the Arduino only draws ~30mA the power dissipation of its regulator will be much lower.
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The Aussie Shield: breakout all 28 pins to quick-connect terminals