I've done a few sessions with 5th and 6th graders. Just 2-3 hour type workshops though, not a full project. Those have been really great.
This Friday I'll be kicking something off with the https://www.laurenceschool.com
in LA. They are doing a project based learning experience around electric cars. The rough plan is I'm coming into the school on Friday to talk for about 45 minutes on car design and my career path to 4th, 5th, and 6th graders....(lots of visuals and movie clips). Then The guys from Icon will be outside with an Icon or two parked on the football field (this should be a crowd pleaser), lastly I'll do a live co-design session, brining a student on stage, interviewing him or her on their family's driving habits, and ideating some solutions.
Post that kickoff, the students have to interview several people, collect the interview data into insights, ideate some solutions, and draw them out. Concurrently they will all get little electric RC kits so they can build them and learn about electric vehicles. They can then modify their kits to take on some of the elements of their design. I forget how many weeks they have to do that, but at the end they are going to present everything back to the team at Icon.
My advice is to make it super interactive and conversational. In my experience these kids have awesome questions and after a few minutes they instinctively get what design is. The questions I have gotten tend to be better than the ones I get when I present at professional design conferences, so leave space for that.