If I were in your shoes, I would advise your h that your children do not want to go to his place. They don't feel comfortable there especially is the OW is there. I would suggest that he give them the time and space to adjust to the entire situation and allow them to figure things out on their terms, i.e., about the OW and that living arrangement over there.
As for telling them that they can't be mean to the OW....well, she is a grown up and should know that kids say and do things sometimes w/o thinking. Your kids are old enough to know what's going on and that this woman has their father's attention. If she can't stand the heat, then maybe she should not be around when the kids come to visit. Your children should be allowed to be themselves no matter where they are at. I would ask for examples of the kids being mean to her. She may be saying this stuff to keep your h's attention on her and keep the kids in line.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
