Clearly you are not on the same page - is this the first time it happens?
Did he confirm the original plans and now changed his mind?
Either way
1. You need to decide if you want to change your plans
If no, why doesn't he go and you stay home with the kids?
If yes, the bigger issue is why you accomodate him when he seems not to care about your needs and wants
Either way, the original plan to take of the gc has changed.
Check with the parents if the new conditions work for them as they need time to make other babysitting arrangements.