Its a very bad idea to jump start your ATV with your car battery. A really Bad idea!
What usually happens when you jump start an ATV or even Side by Side with a Car or Truck is that you fry the electronics of them. The charging systems of cars can do a lot of damage to the small ATV battery and its electrical system.
If You Have No Other Option
If you have no other option and need to start your ATV then make sure the engine is off on your car. Just straight hook up the car battery to the ATV with the Car engine off. This way you keep the charging system from frying your ATV’s electrical components.