This weekend, I played the co-op mode of Portal 2 with out 8-year-old son, and had a blast. The co-op mode is structured in a way where you have to work together, so at the end of every level, both of us felt a sense of accomplishment.
The game has an interesting mechanism for helping communication. When solving a puzzle required him to place a portal in a particular location, I had the ability to mark the location where the portal was to be placed. This made it very easy for him to place the portals.
In addition to the game being fun, we enjoyed solving the puzzles together. I think that once we finish the co-op mode, we will be getting the announced DLC and play that together.