First, I want to say that I do not expect my mother to childmind my children on a regular basis, but I do occasionally ask if she is able to help out in special circumstances. I also do a lot to help her on a regular basis, so don’t feel that I am taking advantage of her.
Two months ago my employer asked me to go on a training course, which would definitely help my promotion prospects (I know there is a higher position coming up in January, which I was hoping to get). I asked my dm if she would be able to look after my children after school 2 days that week until my dh was finished work. She said that it wouldn’t be a problem and put the dates on her calendar.
Fast forward till now, just 3 weeks before my course. I was at my mothers cutting her grass. She has just asked me if I could change the dates when I am away. 
I said that obviously I can’t because it is a set date! She informed me that the friend she was going away with (the week before my course) for a few days can’t now go away on the date they had booked and the company they are traveling with said they could change the date to the week I was going to be away.
Now, I know dm has to have a life outside her family, but I am really annoyed with this, mainly because of the reason for the change. Apparently the friend she is going with has just been asked to collect a neighbours children from school (the week they were originally going away) and look after them until either she or her husband finish work and she has said she will. The company they were traveling with said that they can change to the week I was supposed to be away and friend has agreed to the change, meaning I now have nobody to look after my children, so can’t go on course. I am dreading telling my work I can’t now go and I will probably be given a warning, as the course was costing approx £800, plus over £200 accommodation. I will not be given a second chance to attend course and almost certainly will not be in line for any promotion in January!
I have told my dm how important the course is, but she has just said she is sorry but doesn’t want to lose her holiday. I have finished her grass, but didn’t go in after, as I know we will have an argument if I do. I feel like telling her to get someone else to run around cutting her grass, take her to appointments, get her shopping and all the other things I do for her. AIBU if I stop helping her.