Thank you for your question.
The easiest way for the entire job to be done right when both parents agree to a change of custody and visitation is to draft a joint petition
or motion to modify whatever existing custody order or divorce decree (you do not state whether the parents were married) is in force, according to the agreed-upon terms.
The petition should satisfy all elements of Washington law for modification of a prior order. Material change of circumstances, and how the proposed change is in the best interests of the child, are mentioned in the Memorandum of Law supporting the joint motion, and are "testified to" in sworn affidavits which are attached to the Memo as Exhibits and referenced in the facts statement of the memo. Affidavits are primarily for facts, but opinions are also fair game when they are based on objectively provable facts.
The next document, just to be safe, is a Certificate of Service and/or Waiver of Service for each parent, to inform the court that each of them has received a copy of the motion and memo and its exhibits. A motion to modify in an existing case usually requires just service by mail or hand delivery by anyone who signs the certificate of service. If there has never been a case opened which established custody and visitation and support, then a new case needs to be opened and the Waiver of Service should include specific language consenting to personal jurisdiction of the court. That can save formal service of process fees/expenses and a few days of delay to get the papers (Petition and Summons) served.
Along with those documents, two or more copies of a stipulated order granting the motion is submitted to the court's chambers for signature, along with a courtesy copy of the motion and memo and exhibits. The proposed order does NOT get filed! You call the judge's chambers and find out whether the petition will be granted without a hearing--if not, then you need to go to Master Calendar and get a hearing date and THEN serve a court paper called a notice of hearing on each other, then file that with its own certificate of service.
Many judges require a hearing date anyway so anyone who knows of the petition has one last chance to show up and object (even if they didn't follow procedure and file a written opposition--state courts are more lenient than the federal courts
. It's kind of a "wait and see" thing. If so, we call the judge's chambers and ask whether the judge uses an "approved" calendar AND does not require an appearance at the hearing if there is no opposition. If yes to all, then find out whether the petition is on the approved calendar, show up anyway just in case some relative shows up to put in an untimely objection, and pick up the signed copies of the Order.
File one copy of the Order right away with the court clerk.
Make copies of the Order and prepare a Notice of Entry of Order and serve it on each other, and file that with the Court Clerk with a copy of the order attached and its own certificate of service.
The court usually then closes the case in somewhere between one and eight weeks.