What? Sex in an animal is an act of reproduction, only humans use sex as an act of aggression. You are right about the size of a chicken's brain which is why he doesn't recognize the child as another species, only a being of similar size. Yes, you need to be careful about children around chickens & other potential hazards. It's called parenting. People here tend to anthropomorphize chicken behavior. It is not reasonable to equate chicken behavior with human behavior.
There are many many many many animals who use what is normally a reproductive act as an act of dominance. It is well documented, and you probably couldn't watch Animal Planet for even a single night without running into it. If your statement was true, then female hyenas would not be equipped with false male anatomy that they use to dominate their pack.
Being reasonable with animals is taking the time to study them, to not place our own expectations and assumptions upon them, and to know their limitations. Dominance is a big issue for social animals, and they WILL use every tool available to them to attain dominance, without human limitations of moral concience.
One such limitation is their behavior, and how far it can be modified. With roos... the answer is "not far." Whoever compared roosters to stallions or bulls with extra agility and speed is spot on. Stallions and bulls don't mix with small children as a general rule, and neither do roos. Or turkey toms for that matter. I remember as a child being chased by one, and I ran to my mother. She picked me up just in time and turned her back to him. He raked her legs so badly that she bled. If that had been 3 year old me...I don't even wanna know what kind of scars I'd have today.