I Can't Decide on a Theme! Help!

You could work with an "Out to See Exploration" theme which would incorporate the mermaid and the pirate. The kids could participate in a treasure hunt and you could have treasure map table cloths or placemats with gold coin candies, seashells and nets for decor. They could have to dig in a sandbox to find loot and sift through a kiddy pool of bubbly water to find sea creatures (the sponge expandable pills are a hit with my kids)

If you wanted to grow the "make believe" theme you could involve the kids making their own puppets. Costumes could involve fairies, unicorns, magicians, dragons, and a prince or princess.

Ok I'm gonna go back to the " make believe " theme.
Have a dress up party.
Call it " Let's Pretend"
Have the kids wear their fav costume.

for the pirate, mermaid and treasure party
" Sea Adventures"
And mermaids tie very well into pirates, sailors were always reporting seeing mermaids on their long voyages.

Maybe something like.......

Calling all mermaids....Calling all pirates....

Come help us find some treasure at our Luau!!!!

Luau decor is easy to find!!

Why not just call it a Make Believe party and use the line FArrowood put below "Calling all....Pirates, Mermaids, etc" and then put something about having everyone dress in their favorite Make Believe character. Then tell them they're invited to a Treasure Hunt!! Or just make it a Treasure Hunt party and ask them to 'feel free to come dressed as a pirate or mermaid!"

Sounds like a lot of fun! I'm obsessed with my kids birthdays and we do a big shindig like you do. I swear I've been planning my daughter's birthday (Aug) for months now!

I would go back to your make believe idea and just call it a costume party. Send out the invitations in paper bags with the idea that the child makes their own hand puppet they can bring with them... Have extras on hand for those who forget. Have different a place where they can add things to their pretend friend and a parade of puppets. Since your daughter has her imaginary friends, this will get the others into the same spirit. At the end of the party you can have their paper friends get filled up with treats. Since it is so close to Halloween, this give each child another day with their costumes to enjoy. Who doesn't like to dress up like something/someone else for a day? :)

Pirates and Mermaids, Oh My!

Have you ever seen the cartoon The Backyardigans? They have an episode that is called "Pirate Treasure" and it has a mermaid in it...it is my favorite! It has tons of really GREAT songs in it too! You should check it out...might give you some inspiration?