Here is the post I mentioned about disclosure to children, in Ripken8's thread from 7/3/15:
Originally Posted By: Georgia Bulldogs
Your wife is a wayward terrorist. There is no negotiating or compromising with a terrorist. They will just take advantage of you.
I'm concerned your children. Being introduced to the OM as though he is just a friend. He played a big part in the destruction of THEIR family. OM is not their friend and is not to be trusted.
I feel it is a disservice not to arm your children with the truth about their lives and allow and trust them to function within the truth as they see fit. They don't need all the juicy details but they should be aware of something like "one of several reasons that your mother and I are divorcing has to do with her inappropriate relationship with [full first and last name of OM]. You do understand that it's not appropriate for married people to date other people, right? Well I learned your mother was dating OM and, even though I was incredibly hurt I remained willing to try to save our marriage. Your mother and OM choose not to end their relationship so because of that and other private things between your mother and I we are where we are. I am not telling you this to motivate you to hate and I certainly don't want you disrespecting your mother or any other adult; however, I tell you this because I think it's important to be honest with you because this is your life too and you deserve the truth about what's going on around you here."
If you aren't sure then google information about telling children the truth. They need to truth to protect themselves. It's very inappropriate for you soon to be ex-wife to be sneaking a cobra into the home under the premise that he's her new supportive friend. I know you likely feel pressure to be kind. That being nice is the best way out or through this situation to a place where you and your STBXW can co-parent in ex-spouse harmony; but that's not going to happen unless she respects you. Keeping her secrets only enables more secrets. Be the parent where your children will seek and know truth. God knows, they aren't going to be getting it from their FORMERLY awesome mother.
So, I am pretty torn about things. Advice on this BB can be so conflicting sometimes.