The responses to a situation like this are far too fluid and dynamic to answer from such a general question.
As a father of TWO car seat aged children (an infant and a toddler) it really does make all the difference in the world to have some specifics.
Does the gunfire erupt in front of me, or behind? Can I see the involved parties or only hear the ruckus? Is it close or relatively distant?
All factors that would affect my real-life response.
My infant always goes into the car first, so if she's already belted in and I'm just about to seat the toddler, is my infant between me and the gunplay or further away from it?
How far has my toddler gotten in the "strapping in" process? The majority of gunfire exchanges are over in about 10 seconds or less. If she's already strapped in the best I could hope to do is keep my body between her and the shooters, rather than try to get her unfastened, out of the car, then safely to the ground.
Come up with a quick, specific scenario and let's try again.