For a lot of families, a verbal agreement is fine. A verbal agreement will work if you are reasonably amicable, able to communicate, and trust each other. Also, if you are both prepared to have some flexibility in your arrangements.
Another role I have is what is called an Independent Childrens Lawyer (ICL). The job of the ICL is to present an independent view to the court as to what is in a child’s best interests, and one of the best things about the role is that you can talk to children who are involved.