In the past, I've taught this lesson to 5th-8th graders, usually in a 45-60 minute block. All of the grades love it, and while I imagine that you might need to modify the language slightly for kindergarteners, I think it could still be used.
I teach in a low income area with a high school graduation rate of 1 in 10, and a college graduation rate of 1 in 200. Therefore, I hand out bags of M&Ms slightly comparable to those statistics, although a bit more generous-- I'll give 2 in the class "high school" salaries, and 1 a "college salary." I use the lesson alongside a few lessons on malleable intelligence and the achievement gap in order to stress the importance of earning an education.
I'm not sure what the statistics are for your area, but you could use the national statistics for high school and college graduation rates. At the beginning of the class, I pass out the M&Ms in ziplocks, have them count them, and then allow them to eat them as I talk. I then have them stand up if they have 18 m&ms, 63, etc. I ask them if it's fair that one student got 63, while another got only 18. Then I ask them if it would be fair if every day they came in I gave them the same number of m&ms, and one students ALWAYS got 18, while another student ALWAYS got 63. Obviously, they are outraged by this injustice, and I transition into the lesson on salaries.
In terms of time, I think that 20 minutes might be a bit too short to give the whole lesson, but you could split it into two days. On the first day, you could explain the significance and have them count the M&Ms. On the second day, you could talk about how much money $2.5 million is, and maybe give them a few minutes to draw or talk about what they would use the money for.
Hope that helps!
November 4, 2012