@DrMom,
Basically I told the mom that we had other plans - it was my hubby's birthday (which it was). We had birthday cake that night for hubby - so we neither lied nor hurt some one's feelings.
If it were a larger class it wouldn't matter as the absence of one child isn't noticed. With her small class (5 girls in total) - one not attending is easily seen.
I disagree about only the your child's wellbeing is what matters - this only teaches your child to be selfish. I do think for you as a parent, your child's wellbeing is the most important thing, however, part of your child's wellbeing is to be a good person - if you teach them that all that matters is their happiness, they grow up selfish and self-centered - which will only hurt them in the long run.