Why does my dog hate children?

well my dog is almost a year old. he is boxer mixed with great dane i believe, well a couple days ago, a 6 year old boy was walking down the road with his father, well my dog is usually cool so i let him off the leash to roam the yard. well the little kid decided to look at my dog and that's where i felt tension,my dog got up and stood in front of the kid and fluffed his fur and stood there staring... then then kid made a noise and my dog jumped forward and snarled at him, didnt bite him though, but did lunge forward, then he ran into the road and circled the boy, thank God i know he's trained i yelled sit and he sat instantly, then i grabbed his collar and brought him inside.. now the question. is it his breed? or does he just not like children?