Yes - I'm thinking his also - it seems at least to me from looking from the outside - he was afraid to mention it to you. He was afraid you wouldn't allow it or be mad at him for it. And he really wanted to his grandchild and have your kids go along.
I'm not excusing his actions as you really shouldn't hide something like this - especially where your children are involved - but maybe if you understood how much he really wanted to visit them, it might make the pain on your side a little less.
My opinion is it would be best to talk it out one more time - to get his viewpoint and then close the matter. Maybe say something I like understand how it important it is for you to meet and spend time with your daughter and grandchild, but please in the future let me know the details. I love you and I would understand, but not telling me first makes me feel like you are hiding something and I wouldn't be able to trust you. (or whatever it makes you feel)
Come to a compromise for the future and forgive and forget. Not saying it is easy - but would be better than constantly fighting about it.