Chrissy,
oh they will LOVE it so much! Yes, i think Dreamworld is the best day out because there is so much for kids (and adults!!) you can't get bored. the other parks are great but not as much variety there i don't think.
Thing is, you CAN'T swim at Dreamworld anymore (used to be able to but they took it away) because there is a new water park out the front of dreamworld called "White Water World" which is another admission fee. It's not as good as Wet N Wild though. it's smaller and about $43 pretty expensive for what it is so i don't recommend it...however other people may have different opinions... if you want to swim then Wet N wild is worth it. thats just up the road from Dreamworld and next to Movieworld. Seaworld on the other hand is near Surfers Paradise so about 20mins away east.

Aust Zoo at Sunshine Coast isn't far from Caboolture and they'll have lots of fun there! It's a really good day out... (poor Steve ) it's sad... but you wouldn't see him there even if you went a year ago. i think he was always away on trips.

If you only have a day at Gold Coast then dreamworld will take up most of your day (in fact all of it).

Where else can you spend time at? As in location? Sunshine or Gold coast? and then I can tell you where you could try.