How do I have my whole family work together on a subject if they are all in different grades?

As homeschooling families sometimes we want to include our whole family in on a subject but don't know how to since their are usually so many ages involved. Our zoology unit this year is a wonderful example of how all my kids can be working on the same project, but each at his own level. First Student (12th) is our resident nature expert, so she adds a lot by imparting her knowledge about various animals in our simulated zoo as well as doing her own Biology study. It is enough for Fifth Student (K-4) to color pictures of the animals correctly, learn what habitat they live in and listen to some basic facts about the animals. Fourth Student (2nd), in addition, has to draw pictures of the animals himself, and fill out a form to learn basic facts about the animals. Third Student and Second Student, special needs (6th), in addition, have to write small narrations on the readings or reports about the animals. It is a family project but they are all learning at their own level.