If you have a bag sealer, or Food Saver machine you can put the frosting into disposable decorating bags and then seal them up so the frosting can't come out the top. I recommend a double seal (seal it in 2 places) as I have had it open up after some usage.
Love this idea! I may do this with my kid's next birthday, and I think to sell it to other parties you should definitely have a Caketender there to serve and even squirt the icing, would be SUPER at a Bar Mitzvah!
I'm planning on doing something similar for my sister's wedding reception. There will be a lot of children there. She is already having coloring books made that "match" the wedding party, i.e. the first page is the bride and groom and is says "Melissa & John" beside each of them, the next pages go through each member of the bridal party, etc. We did this at another wedding last year and it was a HUGE hit!
In the same area as the coloring books and crayons, my plan for the cupcake bar is a table that is lower than usual (so that kids can reach it). I'm going ot have vanilla and chocolate (since it's for kids) and just regular buttercream (a couple of different colors to match the wedding colors) and chocolate buttercream, and then sprinkles and nuts. I am putting the icing in bowls with cute little spatulas so that they can just smear it on and then use the spoons in the bowls for the toppings. A friend of ours is going to be the "caketender" and oversee the whole thing.
I saw this pics on flickr the other day.They are already iced,but a great idea.This lady does great work and owns Sweat Pea Bakery in Montana.
http://flickr.com/photos/sethward/2596273177/
http://flickr.com/photos/sethward/2596273177/
http://flickr.com/photos/sethward/2596271665/
Thanks for the links, What a great idea 
I'm going to have to do this @ the next party we have @ our house 
Just to give it a try first.
tenleysmommy.. Great links thanks for posting !
. My other thought was not to use bags if they are going to do it by themselves, but rather a decorating thing like wilton or pamperedchef sell, it has a lid and you pull a trigger.
I agree. I've done it before and even when bags are sealed, most people don't know how to work em. They squeeze all over the place then you have to keep smooshing the frosting down the bag. Too much work. Frosting triggers would work but wouldn't you have to keep refilling? And you'd need more than one, which could get pricey. My solution is to use clear condiment containers. I cut off the tips so they're more open. You can get some that open as wide as a Wilton tip no 10 or 12.
And the cupcake bar is ALWAYS manned by me or a partner not just to offer help or suggestions but to police double dippers, finger lickers, etc
This is a very cool idea, but make sure that you have a cupcake jerk there to do the serving. I can envision some idiot putting the nozzle in their mouth, or dabbing some icing on their fingertip to taste it before using it.
Theresa 
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