Taking DSs, 9 and 11. So both a bit young for real white knuckle, and neither are very boy-boy so 15m drops over water slides? 
.
Choices are:
Dreamworld
WhiteWaterWorld
Sea World
Warner Bros
Wet'n'Wild
I think Dreamworld will be too full-on (and believe me, I won't be taking part in any of the Big 6 thrill rides! Been there/done that/am 48!
Sea World: will perhaps already have seen what's there at Singapore Zoo and Sydney Aquarium
Warner Bros: have they seen enough 'trad' cartoons?
So perhaps the choice is between WhiteWaterWorld and Wet'n'Wild?
What do you think?!