Cookie Decorating Party for Kids!

Hi all,
My name's Jackie and I'm new! But I've been lurking on CakeCentral for a long time.

Now, I have a few questions.
At our local middle school, (I'm in high school)
there will be a carnival, and for community service hours,
I volunteered to host a cookie decorating booth, serving approximately 20-30 (at most) people at a time.
The school is giving me a non-existent ten dollar budget, so along with a partner I'll have to spend some money myself.

At most there will be 400 kids attending the carnival, but usually some do not show up, so I am planning on decorating only 200 cookies or so.
How do you suggest I spend my money the most wisely?

I am most concerned about the cookies and frosting.
I do not have a very big kitchen, and my partner's already baked about 75 cookies, which leaves 125 to me. Would any of you suggest I buy cookie dough or mix, or even completely pre-made cookies?
Or would it be more economical (but more difficult) to make it from scratch?

For my frosting, I think I will simply make a simple shortening/butter based buttercream.
I don't want the kids to have their hands on the food coloring themselves, so my idea was to walk around with a variety of colors for the kids to choose and I'd add the coloring in myself (with my partner)

And finally, decorations.
I have intermediate experience in piping, and the such, so I would be comfortable.
But to keep in mind, the kids will have little to no experience at all, and I do not want to make things more difficult for myself and the kids.
So that leaves candy decorations. I am thinking of buying plenty of sprinkles and maybe dying some sugar multiple colors. Also, I am thinking about buying some popular candies (M&Ms etc)
If any of you helpful folks have any suggestions that would be amazing.

Finally, (I know this is a long post )
how would I bring everything to the school? I think I will ask the school for a few tablecloths or I can bring my own, and the school will be supplying the tables and chairs.
I think napkins will suffice for each student as a "plate" and simple foam/paper bowls to hold the decorations and frosting, along with plastic knives.
If you have any suggestions at all for any of these areas,
that are so very welcomed!

Thanks so much for taking your time to read and reply! You guys are too cool!

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Thanks for all your tips! I've called up a few stores, they'll have to get back to me in the morning.
Meanwhile, I like your idea of the powder sugar glaze. I think I'll make up a small batch of the icing the day before, keeping it a bit thicker and outlining all the cookies.
I have one more question: I know the glaze dries quickly, how would I store it so it wouldn't dry out,
or should I just make it on site? I would make a regular consistency batch for the themselves to spoon into the cookies with the outlines. I think this will keep the cookies looking nice and reduce the messes from icing dripping off the edges.

And the tie-dye/marbling sounds great!
Thanks again, you've been so much help!

Thanks! Well, I just bought a bunch of cookie mix yesterday, I don't have the time (or energy haha) to make everything from scratch.
I also about 8 mini squeeze bottles yesterday, great for filling up with icing.

Now, I noticed your location, San Jose. I'm from San Jo too!