OK, so I'm MumB.
To answer questions/clarify points:
MumA is a good friend, and as a result our kids are friends, us mums made the arrangement to go to soft play. There was no "agreement" to attend from DD, it was more "we're going to soft play with ChildA on Saturday" which she was fine with.
My DD wasn't bored of her friend, she just gets to a point where she prefers time to herself. She very rarely has a personal problem with people, but she gets tired of people's company in general. She does like to have alone time. She will often, after a busy day at school or home, go to play in her room alone for an hour as she wants a bit of breathing space from others, including me and her DB.
I have always always always taught her to speak up when she feels her personal boundaries are being compromised, and not just put up with being unsafe, uncomfortable or unhappy to avoid offending anyone. I'm raising a little girl in a patriarchal world where girls are socialised from practically the day they're born to be polite and compliant and not to ruffle feathers. I'm trying to counter that and raise a girl who will stick up for herself and own her bodily autonomy.
I don't think 8 is too old for soft play, it has target practices and vertical slides, it takes all my power not to join in with them 
I didn't leave because actually neither of my kids were wanting to leave or making a fuss. DD wanted to continue to play, just alone. And because I'm not psychic, I wasn't able to predict that "2 hours was too long" (not that it was) so I'm a bit
at people suggesting I shouldn't have booked such a long session (they only had 2.5 hour sessions)
Anyway the reason I posted is because I'm kicking myself a little that I didn't call my friend out on making my DD feel bad. I guess because I am the kind of woman who doesn't like to ruffle feathers, that socialisation got to me a long time ago (and I was raised to be ladylike and always polite to other no matter how I felt).