If a neighbour or good friend asked you to baby sit on Wednesday night you would say yes if you were able and no if you were not.
However, these are your children and the way she has done this means that to avoid disappointment on your children's part and to stop fueling her anger you are offering an alternative and being reasonable.
Once you take the emotion out of om, using your children etc you can approach the situation in a calm, reasonable way. Mind you it is much easier for an onlooker to see these things, I often don't seem to be able to apply it in my own sitch. I find emotions make you blind.
