I have been using this method since Newlywed posted it in a thread about the best tip from their instructor. It was great for me, but when I offered a cupcake decorating club for 4th graders at school it was a lifesaver. More than 24 kids were accepted before we said "no more!" One of my friends helped me. We did cupcakes from Hello Cupcake and things ran smooth as glass because I had all the plugs made in advance. Think of 24 kids with several different colors. And yes, this is when you need to knot the end of the plastic, because they can not remember to squeeze from the top.
I also do decorating with my friends little ones, yeah 8 or 9 little guys under seven years old, most of them 3. I just make the plugs tie the plastic, use disposal decorating bags and seal the top of the decorating bag with one of those freezer sealer things. Haven't had an icing explosion yet!
I love decorating, but no one has more fun with it than a 3 year old. If you don't have one of your own-borrow one, unabashed fun, they are just so free and creative!