You sit them down, talk openly and don't hold back. Three years ago I was near the end of my rope with my wife. I sat her down and talked, I confessed I was losing my physical attraction to her. I had done everything possible for her to have time to go out for a jog or the gym membership she was paying for but never using. She used the opportunity to also let out her grievances about me, things I hadn't noticed or thought that didn't matter. Right then and there we reached agreements, things didn't change overnight obviously but the work and effort was clearly visible.

Long story short, we are still happily married and last week we celebrated our youngest boy's 1st birthday