All done, all loved it, blood pressure can now return to normal 
Thank you eggrules, it went well. Treasure hunting (stones in sandpits), picnic on the 'beach', pass the Pirate parcel, pin the parrot on the pirate, toilet paper Mummies. Which last was by far the favourite, as they got to throw toilet paper all over the place and go wild: very easy to clean up - recommend!
Then dd1's wonderful godmother came and told a story all about pirates and princesses, involving the kids and giving out teeny gifts that fit the story, ending with the wind (in the story) blowing balloons to them to say thank you for helping. She had them utterly calm after a ton of birthday cake: miraculous.
Now we're back to normal (ie dds arguing, dog eating the sausages, etc etc) and resting on our toilet paper laurels 
Re original discussion: about 3 or 4 mums stayed that I wasn't expecting as their LOs were insecure and/or they had no transport. I had food and drink for them, which they weren't expecting, and helped a lot, which I wasn't expecting.
The big sisters, OTOH, didn't really want to 'help' at all - or go off and play - they just wanted to join in and I didn't have enough props/prizes to go around. That one was a mistake - I thought they'd feel too 'big', but nope! They were a bit disappointed, though as dd1 has her party in 3 weeks am sure they'll get more than enough then 