Check out Youtube for a bunch of kid's songs. Some are from kid's tv shows..
off the top of my head, I'd suggest the one quoted above: A Pirate Says Arrrrrr from the Backyardigans series.
They also have a version of What do you do with the Drinken sailor- although it's short, you could add more verses: What do you do with the scurvy pirate? Make him walk the PLANK!
also- from a Norwegian but English language tv show- You Are a Pirate!
and of course, the old standby: Yoho Yoho A pirates life for me! From Disney.
If you have a flag and something to use as a flagpole, you can have a kid's pirate procession, parading the flag as they sing one of the above songs.
Now, I do kid's parties almost weekly, so I have lots of experience in this.
A couple of tips: Never pull your gun for the kids. Leave it in your belt. This will enhance your "cred". If you have a toy one, this avoids the You're fake! chorus you'll get. I carry a well weathered replica, and it does look real. The parents might get a bit freaked if you are carrying a real flintlock too. The kids always want to know if it's real- I say..does it look real? They say yes- then I say..well- it is real. And they are satisfied. Ditto the sword. Keep it sheathed, and again, I keep mine peacebonded, in case one little powder monkey tries to pull it and attack his friends. It has happened.
I do use my sword to cut the cake. I let the birthday kid hold it, and guide their movements. Caution is advised if your sword is sharp. But it really works well- if the kids think it's fake, and then it's used to cut the cake, and it's a real sword..the kids are really impressed!
There are all sorts of activities- I like to keep the kids busy with outdoor activities. Tug o' war is a real favourite, and I always find time for that. Ditto sword fighting. I use foam swords, but the balloon idea works too!
I also have craft sessions for the days it rains, and I need indoor activities. It's always a good idea to have a backup plan.Designing and making their own personal flags is also a favourite. Making those old style paper pirate hats out of newspaper is popular, and the directions are easy to find on the 'net. There is an version of Tag you can play- called Octopus tag. I of course call it Kraken tag. It involves kids sitting on the floor, waving their arms like tentacles. If they can tag one of the other kid, who are running around, they become the runners, and the taggee becomes the Kraken. I also have a set obstacle course, using boards placed on the ground to walk the plank, as a starting off point, and various other activities.
Another tip: Don't tell the kids what's coming next- just arrange the activities to flow. If they know the activities in advance they'll constantly be concerned with what is coming next, and not focus or enjoy what's happening. Also- let the treasure hunt be a surprise! That enhances the excitement.
I also have a skeleton that I use to hold the treasure chest. so- when they find the treasure, they have to take it away from the skeleton!.it's an additional surprise for the kids and they really love it!
Make your clues on paper teastained to look like parchment. Make 'em piratey but not too cryptic, or the kids won't understand them. I always hide mine in various piratey receptacles...sticking out of an old wine or rum bottle, placed in an old wooden cigar box...in an old brass candle lantern. Make it look as real as possible, and they'll get right into it. I also place coins and trinkets with the clues, so they feel they are getting close to the treasure. Dollar store beads are good as additional trinkets.
Since this is rapidly becoming the Godzilla of postings, pm me for any questions you might have!
Oh yeah: Jamaica Rose's book of pirates has all sorts of cool piratey activities for kids!