Hi Liz,
We went to Aqualand which is about a 15 min drive from albufeira on the motorway last week. There is also one called slide & splash which is a little further away but looking on the websites they both looked the same

http://www.aqualand.pt/algarve
http://www.slidesplash.com

We brought our tickets from the Modelo where they gave you a 10% discount and also 1 child goes free with 2 adults. Buying in advance also meant we didn't have to wait in the queue to get in.
Went with a 5 & 3 year old and had a good day. Had to wait 10-20 mins to get on each ride but if you pay 4euros + 2euro deposit you could hire the rings out which meant you didn't have to queue to get on the rapids and the river but you need to get these early as there is a limited number. Most rides required the person to be above 1.10 m high. My 5year old went on most the rides and as long as they can swim with bands and are ok going underwater will be fine. Took food with us as it was very expensive.

Have visited both water parks mentioned and while aqualand is ok - slide and splash is far better in my (and my kids - 13/11/7) opinion

Cheers Pat O'Hagan

I think Aqualand is more suitable for kids your age, slide and splash has a lot of really big rides for older kids.

have recently been to both and think slide and splash is better for younger children-we had 6 children in our party from 4 to 11, and they all voted for slide and splash over aqualand, which seems a bit old and tired in comparison. Having said that, we had a fabulous day out at both.