If her grandparents are in town to help celebrate, I would let them have that special time with her. I'm sure they will be planning a fun thing or two to do that day. If they don't have anything planned, maybe suggest a few options and prepurchase the tickets.
If this isn't the case and she sees them frequently, then maybe take a half day to go meet-up with them for lunch! You probably don't want to start a precedent for something that can't/won't continue in the future. It sounds like you have a party planned that night, so let her enjoy the day with her grandparents and come home early or meet up with them mid-day!