One thing I have learned through all of this is that kids are so much smarter than we give them credit for. Try not to overcomplicate it with the kids. Reality is that they see right and wrong from your current actions more so than from any talk that you have with them.
Your STBX can talk all he wants, the ow can spew her bullshit as well, in the long run your children will know right from wrong. Truth from lies. Fact from fiction. They will know this from your actions, they will also know this when they get older and understand the dynamics of relationships better.
Try as hard as you can to not really focus on such matters and instead steer the conversation towards how you will take care of them in the future.
Your kids are very bright, they may at times have their vision skewed, but in the end they will see very clearly. Have some faith in that.
