Don't send the letter. I did lots of letters when my husband was in MLC, mooning after OW and had filed divorce....
I eventually realized the letters didn't make a difference to my husband, and if anything it just showed him that I was still there for him rather than detaching and allowing him to figure his own way.
Unfortunately you can't protect the kids from this situation and anything you do say will seem controlling to her. Most likely anything you do will be turned and twisted into something negative so she can validate this situation with OM. Your best bet is to emotionally detach (ASAP!), work on yourself, and try to be a great parent to your kids. If you can stay emotionally stable, work towards being "friends" with your wife regardless of what crazy stuff she's going through (and it does sounds like she may be in the "addicted-to-OM" stage to me --so this could be a real challenge), and try to be the best parent possible to the kids then you willl be a great role model for them.
There is no arriving, ever. It is all a continual becoming.
