Children love to dress up in pirate costumes. If you are planning to have a fancy dress party for kids then this would make a very popular theme for it. You might think that it will limit things a little by just organizing it around pirates but it will actually prove to make it all a lot easier. Instead of having to cater for so many different things, absolutely everything to do with your party can hinge on the swashbuckling theme.
If you have more than one child of your own then you have already solved several problems. You are not going to have to think of ideas for what your children are going to wear. If you are just sticking to pirate costumes then it will make it a lot simpler. As long as each of your children is happy with their outfit then you are onto a winner.
Depending on the amount of work that you are prepared to do (and if you have someone who is going to help), there are all sorts of activities that will fit around the theme of pirates costumes. As far as the children are concerned, if they are dressed up in pirate costumes then they want to behave like buccaneers.
Organizing a treasure hunt for all of the children to take part in would be a great idea. Kids love things like this and it will keep them all occupied for quite a while. You could perhaps design some treasure maps on your computer and print them off beforehand. They don't have to be anything too elaborate as long as they are going to fit in with the rest of the party. You could even tie them all up with sashes and give them to the kids to wear on the belts of their pirate costumes until it is time to look for the treasure.
Plastic swords are very reasonably priced and would look great when worn with their pirate costumes. It might be a good idea to have some 'organized' sword-fight though. If a few children are left alone with plastic weapons without a little supervision then it is only a matter of time before one of them is wounded... Perhaps, to avoid any squabbles, you can have some proper contests for them where they are able to show off their swordsmanship to the fore.
If you have your children's' party in the summer then another idea would be to have some themed games in the back garden. With a bit of imagination, you can soon adapt some ordinary children's activities into something that is more in keeping with pirate costumes. Perhaps you have a sandpit where you can bury some 'treasure'. The kids will love to dig away hoping to be first to find it, especially if they don't even know what it is that they are looking for.
With a little thought and planning (and minimal outlay) you can easily create a whole afternoon of fun that everyone will thoroughly enjoy.