I don't think anyone is saying that ALL roosters are a threat to children, just that they have the potential to do harm and can be unpredictable, especially to children. The roosters I keep are very well behaved around humans, and my children are old enough that I don't have to monitor them constantly around the flock. But when other, especially younger, children come to visit I make sure I don't leave them alone there together.
I think this behavior is part nature & part nurture. There seems to be tendencies among certain breeds to be more aggressive than others. But of course there are exceptions and we've seen enough photos of young children pushing RIR roos in baby buggies to know that. Sometimes frequent kind handling of chicks can influence them to be more docile as adults. Other times it just irritates the bird and he grows up to be more wary than he would have been if left alone. I'm certain that rough, unkind treatment can turn even the most gentle rooster into a wary aggressive one.
As I tell my children, it's not a matter of an animal being "bad" or even "mean", it's more of a defensive tactic made by a fearful & wary bird. People who do lots of carefully planned breeding of chickens would be better able to tell us how much this behavior is an inherited trait.
But to answer the original question: No, not all roosters will be aggressive towards you and/or your 3-year-old. Find another rooster among the many many many others available. Don't put up with aggressive behavior from a rooster. And to be safe, monitor your young children when they're around your animals.
