Hey TT,
My post cam from experience - at BD we agreed that we wouldn't tell our children about the OW and my exH was the one who said that OW wouldn't be in their lives for a few years as he wanted to make sure that they got accustomed to their new life without their Dad being at home etc etc- this was in Oct/Nov 2009.
Fast forward to August 2010 and he was arranging, with the help of my best friend and her husband, to be at the Science Museum in London for lunch and to 'bump into' the OW in the restaurant. I found out about it and made sure that my children were aware of what was happening and that I knew all about the OW and that I was ok with it - children worry about the left behind parent and I didn't want my children to worry about me. It was the worst weekend of my life.
The initial meeting didn't go quite as planned, my son walked off when he saw her saying 'he wasn't going to meet that s**t' - oh dear.... Fast forward 2 years and my kids don't like her, they tolerate her as she is now their Dad's wife, they say she is brittle and boring and two faced. My daughter is very open about her, my son less so but I know they don't see her with love because THEY KNOW the part she played in the demise of their family. I have kept a very dignified silence on the whole situation,being age appropriately honest but never slandering their Dad and rarely the OW.
Kids, at any age, who are coping with divorce or separation need to have some stability and trust in one parent (best to have both but in MLC situations this is not possible). I have simply chosen to be that parent and act accordingly - it is so tough but it is the only way in my view.
be the better, safe option for your kids.