“I have been divorced for three years now an I am living with someone whom I had a child with about a year after I divorced, sadly the baby passed away. The problem is that she does not want to accept my children and treats them very badly when they visit. I love her very much but how do I handle what’s happening?”

Get creative...
This said there is no reason whatsoever to expose your children to unkindness of any sort. This matter is not about your partner or about your children. It is about you making a decision about where you will live, about with whom you will live, and about the kind of environment you will provide for your children when they visit. You can separate, you can find an alternative place to live when your children visit, you can ask your partner to move out when the children move in. Get creative and stop thinking like a victim.