Why did you do your W's dirty work. It was your W's mom that had to cancel on your daughter so it should have been your W that told D7.
How could you have handled that in a different manner?
Personally I would have said to your W.... "W, since this was an activity with YOUR mom I think it's best you tell daughter about the change of plans. I prefer not to be involved with YOUR family plans at this juncture so hang on a second and I will put D7 on the phone".
Her mom flaked and your W essentially told you to not only tell your daughter but left you to pick up the pieces of a heartbroken child when none of this had anything to do with you!
Do you have a webcam? Does your dad have a webcam? If not it might be something to think about. They aren't that expensive. Ask your dad to set aside some time this weekend so your little girl and have her grandparents lunch. Make it special. If your dad doesn't have a computer see if he can go to a friends house or Internet cafe.
Nip this in the bud NOW - do NOT do your W's dirty work. If your W's mom won't have a thing to do with you why in the world would you even consider telling your D7 the bad news?
What crap. Tell us again why you think your W is so wonderful? In the past week alone she mocked you when you did as she said and ONLY spoke about the children by saying "seriously?" when you asked for an official proposal regarding custody for the holidays. She invited herself to a dinner with YOUR family without even having the respect to ask you if felt okay about that AND she had you do her dirty work regarding the grandparents lunch. All that happened in one week.