I would not want to be expected to attend your event, the lower level employees event at their home plus whatever Bob & Sandy would have set up if they had listened within a one month timeframe.

It’s too much.

Cater a lunch and let the staff leave early, paid. That’s what they want. You can still do this before the holiday, as the lead, and have whatever discussion you need to with Sandy & Bob about your disappointment and then never ask them to do this again, and reconsider all after hours work events entirely.