My 2 cents' worth? It changes as they grow up.
When they are very young (up to around 2 or 3 or 4) they don't even realise that other people think different thoughts, feel different pain from them. If little Joey is happy, then it does not even occur to him that anyone else might be less than happy. No matter what he says or does, Joey will never realise that Joey's actions might make other people sad or happy, or cause physical or emotional pain.
The next step is that as the child starts to realise that other people might be somehow separate, it remains all but impossible for the child to imagine what effect his words might have on other people. So he might parrot a phrase Mummy, I hate you not because he hates his mother, but because he has learned that this phrase is a powerful set of sounds and makes his father jump up from the computer screen and take notice.
After that, perhaps around 7 or 8 years old, the child gradually has to learn self-restraint and self-awareness, and how his actions and words affect other people. You can do experiments to show that children start to become good liars around this age. Lying is a social skill. "Little white lies" perhaps, but parents who insist that lying is wrong under all circumstances are doing their children no favours. And the ability to lie means that the growing child has realised that other people have separate minds, feelings and knowledge.
After that, it is a question of age, example, peer group pressure and many other things which govern whether the pre-teen is an awkward, cruel person, or more caring and generally more socially adept. But it is probably fair to say that if a 'normal' 10-year-old inflicts pain--either emotional or physical--it is done with malice, rather than through innocence.
So yes, children can be cruel, but up to age 6 or so, the apparent cruelty is more likely to be the way the child has learned to get the attention. If your 6-year-old constantly says hurtful things, it may be because you have taught him that this is the way to get attention. Just maybe.
Moral: Make sure that you reward your child with positive attention when they are being good, or else they will do bad things to get your attention. From the child's point of view, any attention (even anger), is better than being ignored.