Hong Kong Railway Museum in Tai Po. Take the train up, walk over the bridges across the tiny river and in to the museum. Not a ton to see, but she can walk on a train rail (balance beam) which most kids like to do.
Hong Kong Science Museum in East TST. Just go and let her push buttons and flip switches for hours on end.
Hong Kong Coastal Defence Museum. Ok, more for you to see tanks and canons and whatnot, but kids like it because they can run in the moat (yea, there is a moat) and walk down into the pillboxes and redoubts.
Shek O. Go down, get an umbrella and lounge on the beach. Rent a small inflatable boat and pull them around in the beach break up and down on the waves.