It sounds like he has detached from you and attached to OW. Don't expect much from him as a father. Instead of being upset or bothered just pick up the slack and use the chance to be close to your kids. Being a parent shouldn't be about him, but your own relationship with your kids. If he died tomorrow you'd have to fill that space. Think of him as "sort of being out of the picture." Eventually he may want to come back in because over time he will realize the loss. But for now you are the parent (and just be thankful he's not hogging the kids and trying to get them to love OW).
I understand what you mean about out-of-sight-out-of-mind and how that's not exactly where you want to go. I did do things to be in my husband's life somewhat so he couldn't forget me. But I have to tell you the effectiveness of this is something to be careful about. For one, don't expect much friendhsip when he's still in the anger stage, you can never ever seem pursuing, it must be pure generous friendship, and ALL interactions should be of a positive nature (replacing negative memories/experiences with positive ones). It's a very slow and gradual thing.
Also, you cannot expect this to stop the D, and you must still emotionally detach and move on. Not easy!
There is no arriving, ever. It is all a continual becoming.
