We had a great time doing this trail with our two chihuahuas. They loved jumping over the rocks and logs across the track.
More details on the Ruff Lyfe blog - http://therufflyfe.com.au/dog-hike-review-whistlepipegully/
Fwoahhhhhh. This is some tricky downhill riding - I can see why the locals are so proud of it. The great news is that the park seems to be under development and expansion. Lots of loop options, as long as you don't mind going up the same way most of the time. Would definitely go back; worth the weekend trip from Perth.