Wandering Along the Path: Right Speech

I spent the morning and midday with my foster granddaughter, and I felt I had been accommodating because originally we were scheduled for the Sunday before, but two things came up for her and I said lets not pack in too much.  So her mother said she had nothing on the following Saturday, yesterday, and I agreed to switch.  But Friday in the evening I got an email that they were taking her to her cousin's water polo game, so suddenly another squeeze.  I texted back that the movie was at 9:55 and we were having lunch after.  The mother said no problem.  But the movie was longer than I expected, my granddaughter wanted to quickly get a gift at a store right there, we had slow service at the cafe, and the traffic, because it was Saturday, was horrible.  I was anxious, and when we pulled up, her mother got out of their car and said they were late and had missed the start of the game, said nothing to me, but was clearly angry.  This is after eleven years of being on time on my end, though they have been late and kept me waiting a few times.  I'm offended.  As long as I was picking her up from a short day of school I served a purpose, but now that I've asked to be relieved of that duty, I'm in the way.
I said nothing.  But I will not promise to get her back again on a tight schedule on time.  She lives a freeway away from me and we are usually going a few miles in the other direction.  I am not going to stress myself out because they have the usual harried schedule of events.  I don't feel valued or appreciated except by my granddaughter.  That is why I hang in there for her sake.  I'm hoping some kind of apology is forthcoming, but if that does not happen, I am going to be careful to clearly state that I can't promise to be on the dot.  I did have my granddaughter call and say we were running late, due to traffic and taking longer at the cafe.  Rushing would have been dangerous, and I am a careful driver, especially now that my granddaughter sits in the front passenger seat.  I want no sudden stops or jerky movements. 
We'll see how this plays out.  I'm going to correct my communication to include no promises.  The tension is too much, and my granddaughter felt it more than I did.