We went to Ruda last June, and would definitely go back! the best points of the park is the swimming pool - £1.25 each to go in, the assault course, which my 2 boys loved!
The beach is also great (big waves) not sure if you would go in easter time though!
About Watermouth Castle, it's definitely worth a look. We spent all day there , no problem.
They have interesting museum pieces all round the castle, (old fashioned penny games everwhere) Give your boys a pound each, should keep them occupied for hours!
We went in to see the water show, and enjoyed. There's a mini golf, adventure playground, plus too many more things to mention.
All in all, we found it a good day out, and good value for money, you pay once, and then the only thing realy to pay for is for food, and the penny games!
Make sure you pick a nice dry day, as a lot of things are outside.
Next time we go, we might just stay in the Castle a few nights!
hope this helps