Legal Question in Family Law in Washington

The first answer is that you can't because you would not be the surviving parent. RCW 11.88.080 states it is only the surviving parent who can nominate a guardian by will. Thus, in case you would be the surviving parent at your death, you should nominate a guardian by will.

However, what I have done in the past is to draft an affidavit for my client, signed beforehand and referred to in the will (which must meet the requirements of incorporation by reference), which would address the situation of the other parent surviving my client. The affidavit would request the personal representative to bring to the court's attentions the client's concerns regarding the other parent.

Short of good reasons showing the other parent unfit in some way, your attempt would be unsuccessful.

Your best solution would be to contact an attorney who handles wills, and provide them with a copy of this answer.