I would try and go to AQWA on a weekday or a Sunday if possible, as the Saturdays can be especially busy! If you can manage to wake up earlier, it is also a lot better earlier in the day, as many people begin to arrive in the early afternoon.
This place is really fun for children of all ages, as you can view and touch selected animals in the touch pool. There is also a large selection at the gift shop which can be great to buy things for grandparents, aunties etc
My favourite thing at AQWA is the conveyor belt type aquarium thing. I have no idea what it is called, but it is AWESOME!.
Overall, a great learning experience for the kids and a perfect day out.