You absolutely have to say something, but it must not be confrontational or accusatory. It's difficult to speak up because you're confused and emotional about it. Wait for the moment when your son or daughter-in-law mentions an upcoming or previous trip and just lightly say, "Hey, I'd like to go with you sometime! Keep me in mind for that!"
i've been in situations where I needed to "say something," but it was so hard. The best bet is to keep it light, brief and smile. Whatever happens, just move on. It is what it is.
Could you go see your granddaughter alone, without them? Or take a friend with you? You could spend a little time with your granddaughter, maybe at a time when she is not overly busy and she can give you a campus tour and maybe do dinner with you? and then do some local sightseeing wherever she is. When discussing with your son and daughter-in-law, you could just say I really want to see "Mary" and her school. Keep it light.
In the meantime, keep your own life active, full of friends and other family. I understand your hurt.