I think it's difficult to give you advice. I only know about your children's situation in your ex-wife's new family from your description. Your children would also have to express themselves directly, as well as the new partner and his children. This is all important information that is missing.
Just because you don't like something doesn't mean it's wrong. Children can temporarily experience discomfort and still be happy. It really depends on what the actual situation is in getting the six children together. And there are bound to be problems, even if they live in a big house.
I would maintain contact, but I would encourage your own children to acknowledge the new reality, rather than you - well-meaning - comforting your children, undermining the authority of your ex-wife's new partner. He will also have to raise his voice to your children at some point, to decide that this or that ends or begins. And your children may not like that and will complain to you. But he may have been right at the time, which again, you don't know. Or he could have made a parenting mistake, just like you, but he'd be doubly responsible for one mistake...
That's why I think Sue was right: focus on your new family. Maintain good, trusting relationships with your children, but don't undermine the authority of the new parents. You need help, I think, because this is a very sensitive area.
After all, imagine the same interference in your new relationship, from your wife's ex-partner - if he were to send a lawyer to yours - as others advise you. It only exacerbates the situation...
"What you want people to do to you, you do to them", said a famous ancient philosopher.
It has almost nothing to do with JWs. Catholics, Protestants, Muslims, Hindus, Chinese Communists or atheists deal with the same family constellation.