Oh my.. that sounds horrible.. I mean, why would anyone like that teach? My trainer has only ridden her horse, or a clients horse whilst training me and my best friend - but for us she only needs to give little instructions which she does on her walk breaks - and that is not a lesson we pay for, that is just her free will.
I have given some basic lessons to some beginners, and I would never imagine to ride them. It is always hard to explain how the particular horse works, especially if the trainer has never ridden it and figured out how it works, to explain. Teaching 2 at the same time does not bother me, we get groups of 6 sometimes. However, if A has paid for a private lesson, that should not be ignored - she should have her 30 minutes of just private teaching.
Oh my, and the fact you had ridden? I have a slightly undermuscled unbalanced 5yr old. He was ridden yesterday for probably less than 1 hour by another person, she said she walked and trotted a little and they basically just wandered around the forest (holy mother how hard it must have been for him, mind you he is 15.3 hh and getting fat
) and then I rode him for 1 1/2 h in walk, with maybe 5 minutes of trot at the start and in the end. Didn't even blink an eye, there were many hours inbetween and he actually still wanted to move in the evening when I rode. So ye.. I don't think your mare got too slow or lazy because you had ridden her.
I hope some of the things I am saying make sense.
If I was that kids parent and saw that lesson, I would inform the BM that this trainer is not qualified to do her job. I would suggest finding another trainer my kid can ride with, also I like when the trainer occasionally rides the lesson horse to remind it good manners and responsiveness..
I am just so glad I am out of training atm. Maybe at some point I go to take lessons in the bigger stables, but otherwise a trainer will come to me, or me and my friend work with each other.