I think the wet and wild is not very youth person friendly for my son 's height as he is only 90cm tall…
What about this deal:
https://www.ozbargain.com.au/node/226573
Does family pass have a photo? As sometime dad or papa or granny with take my son to the parks.
Anything else to add?
Given that you live so close to so many beaches, and your son is still so young, I'd say get a bus pass!
Make a plan to visit every beach you can find, not just the ocean beaches between Manly and Palm Beach but also the harbour ones e.g. Fairlight, Balmoral (family friendly) and Pittwater ones e.g. The Basin (catch a ferry from Palm Beach.
I grew up in Manly but we could only go to the beaches that were in walking distance (no car, big family, limited income) e.g. Shelly Beach (family friendly). I preferred the pools over the ocean and you'll find quite a few indoor pools have swimming classes for young ones.
The Merlin pass you linked seems like a good deal, but I'd save the "big day out" parks for when he's bigger. He'll enjoy free/cheap things like a ferry ride, a local play centre, the library, his favourite park or favourite beach for many years to come.
P.S. Message me if you want any Entertainment Book vouchers for your area.