Share battery pack with Arduino and DC motors
You may have read that the UNO can accept 7-12 volts as input power. Your AA battery pack (7.2 volts) is very near the low limit of 7 volts. After just a little discharge of the battery (and also voltage drop from motors running), your batteries are likely to deliver less than 7 volts.  

As your AA battery goes below 7 volts, then the 5 volts (regulated) will also begin to go lower.  A reasonable scenario would be if AA battery was 6.5 volts (1/2 volt low), then the 5 volts regulated would be about 4.5 volts. The UNO is likely to operate ok at this voltage, but as voltage goes lower operation could become a problem depending upon your program used.   

Since you are experimenting with a toy car, I would say to go ahead and run both the motors and UNO from the same battery pack.  Not a lot to lose on trying.  

The UNO already has decoupling capacitors in it's 5 volt regulator circuit.  

Just for fun,  you could use an Analog input to measure your 5 volts and if the voltage drops to 4.5 volts, you could dis-allow motor operation and flash an LED (or other action). Just a low battery indicator.  

If you decide to measure the 5 volts with an analog input,  read up on using resistor dividers and the Analog reference (A-ref). Because 5 volts dropping to 4.5 volts could effect your analog reading, depending upon what you use for Analog reference.