Your agreement is fine all the time you both adhere to it. But as time goes in and situations change, it is not enforceable. An official agreement would just seal it for the future. No need for it to be agressive or to add conflict but just to protect both sides in years to come.
Eg imagine that one day, one or both of you might have another relationship and even other children. The financial emphasis might change and visiting dates might not be so convenient, and the agreement might drift. Its best to get it agreed officially now and while you are both being positive about it.