Read RCW 9.73.030 (Intercepting, recording, or divulging private communication – Consent required – Exceptions)
The statute for recording conversations might be accessed by you by clicking this link
In my state the answer is no. You need the permission.
See my Legal Guide Recording Conversations in Illinois
You need a lawyer because I do not practice in WA (I am in Chicago, IL). Check with a lawyer in your locale to discuss more of the details.
Good luck to you.
Again, check with a lawyer in your locale.
NOTE: This answer is made available by the out-of-state lawyer for educational purposes only. By using or participating in this site you understand that there is no attorney client privilege between you and the attorney responding. This site should not be used as a substitute for competent legal advice from a licensed WA professional attorney with whom you have established an attorney client relationship and all the privileges that relationship provides. The law changes frequently and varies from jurisdiction to jurisdiction. The information and materials provided are general in nature, and may not apply to a specific factual or legal circumstance described in the question.
It is saddening to hear of your troubles with your son's father. In considering legal matters related to children the Washington legal system used the "best interest of the child" standard. I hope you and his father are doing the same. In Washington it is a gross misdemeanor to record an individual without their consent. This is unlikely to benefit you in the underlying case. After all, I fail to see how it does your son any good to call the police on grandma. Further, even without the recordings, it sounds like the information being used was information you told her. Assuming their is some exception to the hearsay rule, like admission against interest, she will be free to testify to the information she knows. She doesn't need the tapes for that. Good luck and I hope things work out the best for your son.
At your service,