Any way to make Behavior tree children?

I want to use them in different characters with some variable changes in task nodes. Is there a way to do this without using a blueprint to change the variables?

Yes. Inside your character, at the main panel under AI, type AI into the search bar (right side panel per default view), the go to the AI class panel and add the AI Controller you want to use. You can make copies of the AI controller and still have the BT stuff refererenced there, and do some custom character specific changes.