Wong Nai Chung Reservoir Boating Park is quite fun - spend an hour or so in a pedal boat on the lake being followed by loads of colourful carp desperate for a feed. Take a loaf of bread (two if your kids are like mine and eat it rather than throwing it to the fish!). Life jackets provided, but you may still like to nail one of their feet to the boat.
The science and space museums are great for young kids too, though admittedly probably not toddlers.
There's a big outdoor water-park/pool complex in Kowloon park that looks pretty good.
Parks in general? Guess you've probably thought of that one!
There are a few books around on things to do with kids - maybe some more ideas in one of those. I'll check the names of the ones I've got later - they're not too bad.
