HopScotchy in terms of clinical diagnosis and intervention it would be a child with an intractable fear/ disgust of primary or secondary sex characteristics, ie severe fear of developing breasts or starting period, or serious discomfort with their penis. That would be a line.
But obviously not just in isolation. Even if it hasn't progressed that far, a little boy say refusing to go to school unless dressed in a female uniform and with a female name, and also insisting they are a girl would obviously need a whole family support approach. I wouldn't necessarily call such a child 'trans' because watch and wait is commons sense. But it would be these things over time taken in to consideration to form a full picture, and decisions would be made as the clinician got to know the child and the family over years.
I'm very conscious of wanting not to to appear patronising with long winded explanations so I'll stop there. But I'm definitely not in support of stereotypes. If a boy wants to play with dolls and call himself Betty it's fine and I wouldn't call them trans and same for a girl being a tomboy etc. It's when it starts to become dysfunctional ie prevents a child/ person from being able to live a happy, emotionally, psychologically/ socially fulfilling functioning life that it is a problem.