Just returned - had really good time but water park was closed.I would recommend staying one night in the hotel so you can check in at11.00sh and go to the park for that day and the next day till 7.00 pm. I would not go more than that. Yes there are some scary rides for older children. The park was almost empty as we chose to go during weekday. We combined it with going to Barcelona for the next 4 days. Plenty of things to do in Barcelona.
We got a 3 day pass when we were in holiday in Salou, used 2 days in the theme park and one in the water park. To be honest I wasn't that impressed with the theme park, and would have preferred to do 2 days in the waterpark, but the ticket didn't allow that.
I felt that we really covered everything in one day and only really went back the second day as we had a ticket. There are a few big rollercoasters and some smaller rides and not very much in-between.
I would say it was fine for a visit, but not sure I would spend more than a day or 2 there.