I am not licensed to practice in WA but you need to see a WA attorney urgently. The following is general advice.
Never voluntarily talk to the police or have you children do so. The police are authorized to lie in order to get incriminating statements.
As to your living situation you need to go back to family court with a lawyer and try for exclusive custody. Clearly things are going on at your ex-wife's that need to be investigated.
It is appropriate for you to be concerned, but you cannot be certain based on what information that you do have that charges will be filed. The case is under investigation. The fact that the prosecutor has wanted to interview your stepdaughter suggests that they are considering the filing of charges, but this still does not necessarily mean charges will be filed. This will depend on the prosecutor’s overall assessment of witness statements and other evidence.
One thing that is certain is that your son has no obligation to speak to the police or the prosecutor to explain himself in any way. He has the right to remain silent. He may choose to speak at some later time but that decision should be made carefully with counsel.
You might also give consideration to hiring an attorney at this stage even though charges have not yet been filed. You could certainly take a wait and see approach. However, an attorney may be able to determine where the case is headed i.e., charges or not, and may be able to advocate on your son’s behalf before a charging decision is made. While it is nice that the detective made such reassuring comments, you cannot be certain that the comments were genuine or that the detective’s beliefs will control the person who ultimately makes the charging decision, the prosecutor.
DISCLAIMER: This is NOT legal advice and is not intended for the reader to proceed in reliance on this information. The reader should always consult with an attorney regarding these specific questions as they pertain to the specific facts of his/her case.