How to tackle this depends a lot as I see it as to whether you think your husband was right or wrong in what he said,
I am unclear whether he had words with your daughter, her children or you.
If you feel he was right, the only thing you can do is write to your daughter saying you are sorry their visit turned out so badly, and that you hope she and the grandchildren can move on and let bygones be bygones.
If you feel your husband was wrong, then you need to get HIM to apologise to your daughter and grandchildren whom I assume are his daughter and grandchildren, too.
If he won't either apologise or let you do so on his behalf, tell your daughter that you too are cross with him about what he said, but that you cannot change his mind about it.
It is unfair of her to take it out on you, unless, of course, you sided with him, but from what you said, you were not involved in what he said.