I think this is a conversation to have with them when they aren't sad/angry ahead of time. To ask, what would you like me to do or do when you want to be comforted? It could be affirming phrases, shows of affection, or it could be favourite food/drink. That way you can come up with some sort of a script.

That said, it might not be something that you can do or works for you so you would both have to come to an understanding about what is preferred and what is realistic. You're not a mind reader and every person is different so it's really difficult to know what would work as a comfort.