My puppy lunged at a child

My 6 month aussie/heeler mix is really weird.

He barks and lunges at other dogs, growls, barks and yaps (but has never bitten). Also reactive on the leash. I'm watching Cesar Milan videos and have really helped. He is usually very reactive on the leash but after the video he ignored some dog passing by.

When unleashed at the dog park, he lunges at other dogs until they leave (very territorial). But one time he socialized fine with two elderly dogs.

Yesterday however, he was unleashed at the beach and played fetch just fine. He ignored everyone passing by including children.

But today we unleashed him at the local park and lunged at a kid who ran away all scared. He only lunged until the boy ran away as of saying "get out of here"

Why only children?? He loves adults.