Wicksteed Park is the place. It's superb for young kids, especially if the weather is half decent. The adventure playground even has a nautical theme, with rigging, a giant octopus and so on. It's one of the biggest playgrounds in Europe.
The pirate ship may be a bit "grown up" for a five year old but it is right next to the playground so adds to the theme.
The boating lake is good fun (as long as you can row!). It's quite large, with an island you can row around.
The boating lake and pirate ship are only open on high season days, shown in green here:
http://www.wicksteedp...uk/Opening-Times.aspx
The adventure playground is open all year around and is free...