A camping theme is great for kids slumber parties.  You can also take some of these suggestions below and make them appropriate for a non-overnight party.

Invitations

You can make them using clip art on the computer or have the kids make them by hand.  Send out a couple weeks before party.  Make sure you give address and directions to party, as well as phone number for parents to call to RSVP or if they get lost.  Don't forget to tell the invitees to bring sleeping bags, pillow, clothes, and anything else they will need overnight.

Food

Lunch or Dinner: Hot Dogs (can be heated on stick over fire)
Breakfast: Bagels (can be toasted on stick over fire)

Snacks:
Marshmallows
Chips
Fruit (fruit cups or canned fruit, apples, oranges, or other fresh fruit)

If you are having the slumber party to celebrate a birthday, you can also have cake or cupcakes, as well.  Decorate them with choclate frosting to look like mud add candy bugs and/or gummy worms.

Decorations

You don't need to really decorate in the way you would for another kind of party.  The main decoration is going to be setting the scene to represent a campout.  The best way to do this is set up a tent outside and make a place for a campfire.  You could also set up a tent inside if you have space and use a fireplace as a campfire.  Lawn chairs also make a good addition to your "scene."  Also, provide flashlights and glow sticks (you can get good deal on these at most dollar stores) for after dark.

Activities

Firewood Collecting:  Unless your backyard is full of suitable natural firewood you will have to buy some.  Before the attendees arrive scatter it around your backyard.  When you are ready to start fire have the kids find the wood and help construct the wood into a teepee formation.

Cooking over Fire:  Provide sticks (wood or other ones found at camping stores) for cooking hot dogs over fire.  They can also be used to toast bagels and roast marshmallows.  Provide chocolate and graham crackers to make S'mores with the roasted marshmallows.

Campfire Stories and Songs: Let the kids take turns telling campfire stories or tell some scary stories you know.  You can also lead them in campfire songs such as "Kookaburra," "The Tree Song (The Green Grass Grows)," and "The Ants Go Marching."

Scavenger Hunt:  You can do a hunt with vague clues like find something green and find something round.  You can also give descriptive clues to try and get a specific items such as find something green, round, and spiky (a real thing we once found camping pictured below that was strange to us).  The clues can lead to things that already exist in your backyard or you can 'plant' things to be found.