One issue that preservice teachers can have with addressing the idea of algebra in the elementary school is their perception of what algebra should look like at this level. I suggest beginning this topic by asking the PST a simple question:
I can't speak for all PST and their responses, but I'm confident that you will hear a lot of ideas from a high school algebra 1 course. If the discussion needs redirecting, try and focus their attention on some of the big ideas in algebraic reasoning:
This site contains some GSP files that can be used to address some of these larger ideas in algebra. Each file is a dynamic environment that can be used for exploration within a given area. If you click on the links above, you will be taken to a page that contains the file and some of the possible instructional ideas afforded by it.