Start here:
125 people divided by $500 - $25 for the place = $3.80 per person including paper products
Before leaving home call around and see what you can find for costs on paper products, trash bags, food items, etc., even Walmart, Big Lots, Patry Stores, etc.
For tablecloths(or picnic table cloths)you can take local newspapers(better if she is interested in sports or music)and lay them out on the tables, then cover with a roll of clear plastic from Lowe's. You can save the plastic for uses at home later.
Look at sectional styrofoam plates for eating, with water(unless you are serving it out of a punch bowl)and pop they come in their own containers. If you are buying the large liters of pop to serve, make sure you have someone doing the serving. And serve in 6 ounce cups, ice provided if preferred.
Buy your balloons but call around and ask what stores, like Dollar Tree, would charge to inflate 1/4 of the with helium, then attach 5 or 6 to 1 of them to keep them "afloat" in a group.
Once all that is done, you can work on the food, the least expensive part of your budget.
You can make the subs ahead of time, so buy your deli meat in single roast form. Have them shave it(if you cant do this part at home), you can assemble and freeze till party time(take out to thaw the night before). Just lightly butter both sides of the bun lay the meat(and cheese) into them, wrap and freeze. Probably 2 per person.
Make your buns yourself or again call the local bakeries and stores and price out the sub buns. Dont forget to contact the bread stores in your area also.
Any vegetables you want to add, you can slice and shred at home. Tomato, lettuce, onion, etc.
Cheese is expensive also, so use one flavor or type and have it sliced from a bulk piece.
Make the salad the day of, preferably at the park immediately before serving and keep the disposable pan in another pan of ice. Tossed salads can be very tricky healthwise at picnic events. And you will also have to serve dressing, big cost over say a pasta salad. I would combine the salad and the condiments for sandwiches together to let them choose which(or both) they would rather have.
Buy all your pickles in the largest jars you can(gallon), if using one type place into a bowl with a fork to serve.
Make your baked beans at home one large disposable roaster should be plenty.
If you can buy the mini bags individual bags of chips(or rebag the large ones in nice cellophane or paper treat bags with the school colors), serve in paper or plastic cups. Easier to handle, better portion control, and easier on the budget.
Pop and water, serve in seperate iced containers, make sure the water is served at the front of the food line and the pop at the end of the food line, even after the desserts. Cheaper, healthier and more convenient for parents of little ones. : )
Make the cakes yourself, even if you make 5 or 6 different styles and set them out in a nice display, cake is cake. Make one decorated nicely for her or order it, but the rest of them you can make ahead. Same with the frosted brownies, depending on the school logo, you can bake them in sheet pans and cut them out with cookie cutters, serve them individually on paper coffee filters(large cupcake holders really).
If you need a time line and any recipes let me know would be glad to help. : )
Brianschef Michelle