i have lived in sydney my whole life and have 3 kids so i will tell you my opinion.
sydney sux lol in terms of attractions. taronga zoo, is always having work done, half the time the animals aren't even there and it is very big so it's hard to see all inone day, and the kids get tried.
aquarim/wildlife park are ok, nothing special but i personally think way over priced.
sydney tower is ok, once again nothing special. it is basically a look out.
personally i think you are better off doing things like-horse riding in centenial park, botanical gardens/opera house walk,sydney observatory, exploring the rocks,ferry ride or cruise of sydney harbour, exploring cockle bay, imax theater, urban jungle adventure park.
that;s all i can think of now. these things won't cost you an arm and a leg either.
The day i broke up with normal was the first day of my magical life...