When I started working with young children, when I would do a unit with a recycling or environment theme, this would always be a project of ours.
At the time, I think it was borne somewhat out of my own habits and also a desire to just... get them started. When you really think about it, small children have little power over the very large damage that is being done to the environment, but this is one small way to get a good habit started. If you give them that, and they feel like it is a responsibility they have, then for many, this will just be one of many habits that they develop over the course of their lives.
I know that this is how it worked for me. As an adult, I am very aware of the things I do and how they affect the environment. I drive very little and the car I drive is not a gas-guzzling SUV. I recycle way more than I put into landfills and I use services like freecycle to keep the BIG stuff out of landfills as well. I am constantly educating myself about ways I can do even more. As it is, compared to most who live in the United States, my footprint is pretty small (8 acres compared to the average 24, but that's still too big, I fear...)
So, while it may be true that something small and easy like snipping six pack rings doesn't do just a whole lot for the environment compared to the big, hard things... for kids and maybe for their parents who aren't that "in the know" about environmental issues... it is a start that could translate into a lifetime of good habits.
So include this activity (or something similar) in your lesson plans that have to do with the environment or Earth Day, etc....