Today is Cinco de Mayo! If you're still in a bind for what to plan for your party, I wanted to give some kid-friendly tips here:
Pinatas - A great game for children. Fill up your pinata with candy, and lets the kids go at it. Another way to use pinatas are as party favors - give each child a mini-pinata to take home.
Decorations - There's a variety of vendors out there that sell Cinco de Mayo themed decorations. Choose festive colors, tissue paper, etc. This vendor is an example (http://www.windycitynovelties.com/1710c/cinco-de-mayo.html) - if you need it right now, head to your nearest party supplies retailer.
Food - The follow are possible suggestions that you could have for your party: Sweet Mexican corn cake, nachos, taco salad; anything to give your party that certain flavor!