In reply to kathrync:
It might be worth a trip to cyclopark
http://www.cyclopark.com/ one day soon. It's near Gravesend, but worth the drive. Smooth, grippy tarmac, no cars, no potholes, bends, hills, flat oval etc. An hour or two pedalling around there, with your niece too, would be a few hours well spent for both of them. There's also a mountain bike trail - undulating and not technical at all. Café's good too.
As a club we go there periodically. It's very good for confidence and bike handling generally.
It's where I learned that two drunk cyclists can ride their bikes home from the pub by leaning on each other to prop themselves up.....