I love going to the Blue Mountains, been three times in the last year and a half. Twice to see the Leura Garden Festival and bushwalking and another time solely for bushwalking.
Although I don't live in the area, I can tell you about some of the places I went to. The gardens you are probably thinking of are the Everglades, it costs about $6 for adults and $2 for children. I've been there twice now in the spring and it's a very nice garden - very big too. Plan to do a lot of walking in it. Great photo opportunities. Lots of azaleas (mainly azaleas), australian bush etc.
Are you planning on doing some bushwalking? If you do, the Leura Cascades is a nice easy bushwalk. We took a few kids on that bushwalk and they had fun. There is a waterfall that runs the length of the walk - not stunning but a waterfall nonetheless. I can't remember the other bushwalk we did but I do remember that it was a lot harder but it had a good view of a plunging waterfall. It was hard because there are sheer drops left, right and centre, not a good place for kids.
The Wentworth Falls Lake is a very pretty spot for a picnic. There are plenty of ducks to chase after
It also has a swing set for the little ones and toilets nearby. Also nearby is this little cakes and sweets store (across from wentworth falls station) that a friend told me about who used to live in Wentworth Falls. I haven't tried it yet but she said they are absolutely scrumptious. If you try anything from there, do let me know how the deserts are
Here is the website I visited often when deciding what to do and what to see:
http://www.bluemountainsonline.com.au/
My experiences in the Blue Mountains have been great every time. I hope you enjoy your stay there Mike.