I just came across this Web site today via a Facebook group. Here it is: http://tryengineering.org/
It is filled with over 130 lesson plans for various engineering projects for kids! It seems to all be free too! 
We are exploring engineering with our 7 year old this school year via a Beginning Architecture homeschool curriculum, but this site seems to have a lot to offer as well. There are lesson plan PDFs, printable student worksheets, etc. I think this could be used really well in a co-op setting as well. Most lessons seem to be designed for ages 8-18 from what I can tell. When you search by category of lessons, you can leave the age range blank and it will pull up everything in that category. I hope someone else finds this useful too!