Under no circumstances should any child, especially one who has lost both their father and mother, ever be told to their face that they aren't part of their new family. OP, what your FiL said cuts so deeply, that I don't think there is any way I could ever consciously allow them any interaction with my family for the foreseeable future. This is something that absolutely qualifies for immediately cutting them off for the rest of your life.

Your husband is in a very shitty position. I think you need to explain to him just how devastating these words were. His own parents have basically spit vile hatred over his soon-to-be adopted children. This is inexcusable behavior. I would explain to him that part of being a parent is protecting your children from such ugliness. This is something that will have left a scar on them that may never truly heal. Maya, in particular, will always remember this moment, and that's something truly sad. I get that he doesn't want to lose his parents and cut them off. That is a big decision. It's also an opportunity for him to show these kids that he cares for them and their well-being. Bringing them around more often is the WORST idea. It will only expose them to more and more of this unnecessary hatred and it will really impact their lives.

I think that there needs to be, at minimum, of 1 year of no-contact. At this point, you should both reconsider and talk about it again. And no contact includes Kevin. He needs to stand with you and for the kids with this. They're glad to not have to deal with you or the kids, but them losing their son is what would impact them, and that is what needs to be stressed.