It doesn't look like anyone tried to answer jdurg's "hazy memory of the situation". To put it simply - there was not much information available to remember. Going deeper, the brain tends to categorize visual information according to importance/relevance and ignores everything else. When you're driving, you're usually allowing your well-trained subconscious mind to control the vehicle using visual information of the road, while your conscious mind (i.e. you) is thinking/visualizing something else - e.g. things to do at the destination, someone talking on the radio, the stupid sun in my face, the shady guy with a knife in the back seat, etc.
The subconscious mind is like an extremely quick pattern-matching machine which finds patterns from all the sensory devices (visual, aural, touch, smell, taste) and performs either an instinctive or trained task, or notifies the conscious mind to make a decision (which is much slower). So, when you spotted something dart out in front of your car, your subconscious mind rapidly hunted for all possibilities with the limited information it was given - "it's a moving object... has to be something that's alive... a dog? a child?" A kid getting run over, or almost run over by a car is not an uncommon visual. You have probably seen plenty of it in movies. So it is likely that your subconscious mind associated this experience with that memory. The instinctive/trained reaction is to hit the brakes and swerve away. At this point, your conscious mind should already have been notified. I'm betting that it was a conscious decision to swerve right rather than left.
Back to the hazy memory... only the conscious mind is capable of recording new information and creating memories. The subconscious later prioritizes and catalogues this information during sleep. As far as we know, the sensory information fed directly to the subconscious cannot be remembered (or is very "hazy"). Hence why most people cannot remember their dreams or have a very hazy memory of them. In your case, your conscious mind participated very little in the whole event except for the aftermath. Notice how you seem to remember your decision to swerve right - a possibly conscious decision. Also, you remember thinking that it might be a kid - another memory where your conscious participated. Almost everything else was subconscious - hence hazy.
Phew! That was a long post! Sorry I couldn't make it shorter.