Why don't you put him in his costume the day before and take him to Grandma's if it's going to be a sit and visit situation? Otherwise, I think you should have popped in so she could see him, excused yourselves to go trick or treating, then come back there after for a visit. Depending on the age of your son, this could have take 20-30 minutes max to hit a few houses. I don't think you should blame your MIL for the length of your stay. I'm sure she had no predetermined time in her head, especially since it was you and your husband's idea to go.