I just had this conversation with my son last week. My grandson (his son) had open house on the opening night of bowling and son was going to go to that. I told him that he had his priorities mixed up and that going to the opening house took precedent. I explained that it told his son that he was important and that you can go bowling any old time (his birth mom was not going to attend the open house never does). He said that he did go to the open house that night when I talked with him over the weekend.
It's nice to have a birthday celebration but there are going to be times that it is not going to work out and that is part of life. So once in a while a little change will do good to help them become flexible adults. Besides it's not like you aren't doing anything for her you are having a party early.
Enjoy the open house and meet the teachers.
The other S.