There are NO forms you need to fill out in Missouri. If your child was in public school, you only need to write a withdrawal letter. Educators will tell you that you must register with your district, but you DO NOT have to, and all the homeschool orgs discourage it.
Yes, you are right. In Missouri, we can opt out of a Declaration of Enrollment (section 167.042, I think). However, many states are not so laissez-faire about it and require forms.
Cheers!