i dont think its possible to make existing blueprints children of others using blueprints. maybe in c++ you could do it though. i would have made a base character class then made your individual characters children in the first place or made them the same blueprint and had variables for things like the mesh and their name.
i believe the technical reason you cant connect them both is because they inherit from different blueprints that makes them different variables even though they have the same name and type.