Many clients in similar situations overlook the simplest solution: speak with the other parent and get their agreement. Remember: they are going to find out about this change in your life anyway and it is often better if they hear about it from you first. There may be good reasons you have sole decision making authority (like abuse or neglect). If that is the case, you may want to have this disclosure only in writing. Assuming that the other parent is not willing to agree to your proposed solution, there may be steps you can take to resolve this issue before your reporting date.
If you can agree, I would strongly urge that your agreement be in a written, notarized document. Again, the document does not have to be long and in "legalese," but it should be in writing so that everyone understands the situation.