West Wickham to Tonbridge by bus isn't going to be a pleasant journey for your dd, I would imagine that she will have to change bus at least once. Is there any chance that you could drive her to Bromley station? It would make her life so much easier especially with all the adjustments she will have to make changing school etc. Her journey is going to have to start very early if she has to go by bus.
Your first point of call should be the KCC website and the school transport option. On that page is this journey planner link
http://www.travelinesoutheast.org.uk/se ... eOffset=15