Your local firehouse should be able to provide you with some fire safety activities that you can either use as decorations, or include in the goody bags. Some also have plastic fireman hats and fire marshal badges that they're willing to give you.
If you have a stuffed Dalmatian, put a fire hat on it and put in near your front door to greet the guest as they arrive.
Make a banner that says "Welcome to (your last name) Hook and Ladder!". Hang in on the front door, or in the party room.
Use red and yellow for decorations, streamers, balloons, table coverings, etc.
Your local fire house may be willing to bring a truck to your home the day of the party, and allow your guests to climb in and have a look around. Just keep in mind that they may have to leave on a call, or may not be available if a call comes in before the party. It's best not to tell the birthday child or guests that the truck is coming, to avoid disappointments. Consider making a monetary donation to thank them for coming.
Make flame shapes from red and yellow poster board. Put the flames in your windows, and around the party space.
Use any toy fire trucks your child already owns as centerpieces and table toppers.
Put a yellow rain slicker on the back of a chair, with a home fire extinguisher sitting next to it.
Use new, clean, red and yellow pails to serve chips, and to hold napkins and flatware. Decorate the pails with firefighter stickers, if desired.
Use a toy fire truck you already own as the centerpiece for the table.
Serve hot dogs, chicken nuggets, chili and grilled cheese.
To go along with the cake, serve Dalmatian cookies. Make bone shaped sugar cookies using a cookie cutter. When cool, frost with white icing, and stick in upside down chocolate chips to make spots.
To make a firefighter cake, you can use a fire truck shaped cake pan.
Another option for the cake is to make a fireman's helmet. Bake a rounded cake using an oven proof bowl, and one round cake. To make the bill of the helmet, cut a crescent shape out of the round cake. Discard the rest of the round cake. Fit the crescent snugly against the base of the domed cake. Ice the whole thing with red frosting. Make three crisscross lines across the top of the domed cake with dark colored get frosting to look like the markings on a fire hat. Attach a plastic fire chief's badge to the front of the helmet.