I received an invitation to a grandchilds Siddur play
First of all, mazel tov! May he/she continue to grow in avodas Hashem.
I think you’re being too harsh when you call this a lack of mentchlichkeit. Not everyone considers attendance at a siddur play as important as you do. Your difference of opinion with the school is probably in this regard, not that they think you should take off from work in order to attend.
You can certainly call the school and share your concerns, and if enough parents and grandparents do so, they might reconsider.