Dreamworld would be good as from memory it has little kids rides as well and movie world. Wet n wild might be ok too...lots ofpools and slides for different ages.
In terms of cost - you only pay to get in the gate then go on rides as many times as you like (there will be other little things around the park you might have to pay for if you want to do them) but all the big "normal" rides you dont then need to pay for.
We used to take our own food (shove it in the nappy bag )

Are you an NRMA member? I know they do superpark passes not sure how much cheaper it is through them but worth checking it out

HTH xx