I looked in my copy of Jean Greenhowe's Party Costumes for Kids. This gave me some ideas for putting together a pirate costume. Most of the basics - striped top, bandana handkerchiefs, striped socks, belt with buckle - were already to be found in our house, but I had to do something about the trousers.
Rather than sacrificing a pair of ds's existing trousers, I bought some skull-and-crossbones printed fabric in John Lewis. Using the method from Sew Hip issue 7, which I previously used for my mermaid pyjama pants, I drafted a pattern to produce some simple long shorts, with an elasticated waist. It took less than an hour to stitch and fit these.
ETA: Here is ds wearing his pirate outfit, all ready for the trip.