Rational Jenn blog post: Homeschooling and Perfectionism published 12/19/11
I look at this differently than you approached it. To me this is about making informed choices and accepting responsibility (yes even for a 6.5 year old).
I would have said to my child, "If you sign up for this co-op class there is homework this is like this (describe it) and you must do it as it is part of your responsibility as a student and the teacher feels it helps form your experience in the class in some way. Whether you like the homework or not, I don't know but, it's a part of the class that you will have to do."
In my favorite co-op the homework estimate was put in the course description so there was no surprise. Some kids chose to not take a class as they liked the topic enough to go sit in the class but didn't want to do homework as they had homeschool lessons assigned by mom to do on non-co-op days or they liked the idea of the class but didn't want to do more work. And they didn't want surprises after enrolling in the class and starting to attend.
(Although in your daughter's case she was doing most of the homework, the reading.)
If you found out about the homework after attending then again present it as an option to do the homework and stay in class or drop the class.
I think the assignment was easy and very do-able for a 6.5 year old especially if you helped by typing out what she dictated to summarize the book. She also could have kept the books read log in Excel in an easy list format, or Word if she wanted to avoid trying to make perfect looking handwriting by hand.
It may seem small to you but if you present everything as optional and open for their rejection, even to young kids, they grow up with an attitude that their opinion or whim or desire comes first and foremost ahead of things like obligation and responsibility to an outside party (i.e. teacher, authority figure, boss, police, friend,or spouse and anyone).
People who abide by the law, who are good employees, who are good neighbors, good friends, good spouses, and good parents often have to jump through some hoops or be bored or do things we may not think is what we want in order to just live through daily life in a civilized manner.
"I really don't want to clean this barf that my child just did on the rug but for the family's health it must be done."
"I don't feel like stopping for this red light but I need to so the other cars can go."
"This meeting at work is really boring how can the boss think this is worth my time, I feel like walking out, but I can't."
"I did want to finish watching this YouTube music video but my brother needs the computer to do his math work so I'll get off it now instead of in five minutes."
I am a perfectionist too, and I understand that whole part of the discussion, and others have already said good things about it so I am not addressing that much. I think you are handling that part well.
I will add:
Perfectionists need direct instruction in time management so they do not continue to push off a task. There is a fear underneath (they may not realize) that they won't do a good enough job so they put it off. Then that gives them stress, facing that deadline and not having it done. Often they don't do as well on school assignments as they run out of time.
(There is actually brain science behind this, the brain which is anxious has different brain wave activity that impedes things like congnitive thinking ability, memory recall and other thinking tasks that would make it harder to perform well to write a paper, summarize one's thoughts in writing, or take a test.)
If you can directly teach your DD this and get her to face that fear and push through and get to the other side, and use planning and time management early, she may bypass years of repeated action and failure before she discovers that fact herself.
It seems to me you want your kids to figure out a lot on their own and arrive at their own conclusions. Sometimes a little nudge by Mom can really help kids rather than letting them flounder for perhaps years. You have wisdom and I feel that parent's role is to impart their wisdom to their kids.