Use toys you already have on hand to decorate the party room, stuffed horses, cowboy hats and boots, and toy trains.
Use a red and white checked table cloth.
Decorate the tables with mini bales of hay with red and blue balloons tied to them. You can get the hay at the craft store.
Make signs the say Welcome Buckaroos, Welcome to (your child's name) Rodeo, Park your horse here, and No fightin' and no spittin'. Hang them around the party room or throughout the house.
Make cowboy vests out of brown paper grocery bags for your guests to wear.
Make and hang wanted posters for each of the guests. Give them aliases like Naughty Nick, or Fighting Freddie. Under the "wanted for" section, make up goofy crimes, like laughing to loud, having too much fun, or being too smart. Let the children take home their posters when the party is over.
Using a refrigerator box, make a jail. Paint a brick design on the outside, and cut some bars out so the children can look out. Decorate it with some of the wanted posters. It will be a fun spot for taking pictures during the party.
Use bales of hay as seating. You can buy them at your local nursery.
Play western music. The theme from The Lone Ranger or Bonanza would work well, as would a Roy Rogers CD.
If it's available to you, some riding gear would work well as decorations. Saddles, bridles, horseshoes and cowboy hats.
Serve food in small aluminum pots, or on tin pie plates.
Serve "camp fire" food, such as chili, hot dogs, baked beans, grilled burgers, and cole slaw, chips and salsa and chicken drumsticks.
To go along with the cake, serve s'mores. You can buy them ready made, or if you have a fireplace, make them yourself. It's best not to let the children make them, for safety's sake.
Make "cactus juice" using green punch and ginger ale.
Make or buy star shaped cookies, and pipe "sheriff" on them with icing.
For a cowboy birthday cake, bake a chocolate 9 inch x 13 inch cake and frost it with vanilla icing. Use upside down pretzels twists to make a corral fence around the top edge of the cake. Use green sugar for grass, and shredded wheat to make hay bales. Crush up some graham crackers for dirt, and put some plastic horses on top of the crumbs.