The first week of kindergarden is one of the most important times of a child's life! It sets the stage for the rest of the year, starts good habits of attending school and later work, gives the child respect for her obligations, and orients her into the particular school, class, and teacher. She should start out with the idea that school is fun! and important! The child will get a lot more out of that first week of school than too much time with grandparents and cousins, especially since she can go for part of the time. I am appalled that any responsible adult would suggest the child start off with the attitude of blowing off something like that. But, you already said no. (You hit a real hot button with me on the grandparent's suggesting skipping school.)
You just need to keep saying no, and now that you have explained it all to them, quit justifying.
(And best wishes with the new little one on his way, and hope your daughter loves school!)