Children and young people
There can't really be trans children, can there? Kids can't know for sure how they feel when they're really young, right? Wrong. Gender identity is thought to be solidified by age 6. This does not mean that children absolutely, positively know how they identify by that age.
It simply means that their gender identity is there. If it doesn't match up with the gender they were given at birth, then that will start to manifest itself in different ways. There are organisations like Mermaids that support families and young children and young people.