My advice is about the pinata...
We long since stopped using the store bought kind you hit with a stick since we started making our own that the kids kick (takes about a week to fully dry).
Use one of those big round balloons that you bouce with a rubber band as the mold. Do a layer of paper mache (tons of fun for the kids to help with). Let it dry and apply another layer every day for a week. Fill with candy on about day three and patch the hole with the next layers. Paint however you wish when dry...
Let the kids stand in a big circle and kick the pinata ball back and forth until it bursts. FUN! A variation is to make several smaller pinata balls for the same idea.
I would also encourage the kids to have a unisex or one themed party. If you are doing it together why have two themes going?
Any games involving bubbles have always been a hit at our parties, including putting the kids one at a time in the center of a baby pool filled with bubble solution and using a hula-hoop to trap them inside a bubble. Take a picture and send it with the thank you cards.