If her main request is to see MSM, then you should do MSM the right way. The "wrong way" to do MSM is to try and do it as a day trip from Paris. While that's an option, it gets you an experience at MSM that leaves many disappointed. because....crowds, severe crowds. MSM is world famous. Package tours disgorge thousands of their customers on the doorstep of MSM every day. It's important to remember that MSM has one small, short, narrow street - that's it. During a few hours at mid-day, everyone squeezes in to that small space, and it's extremely crowded. But the crowds are only there from late morning to md-afternoon - the Day Tripper Hours. Before and after mid-day, the place isn't badly crowded. Early morning, and into the evening, the place is all but empty - and magical. The MSM of your dreams (or your granddaughter's dreams) is there, just not from 10 am to 4 pm.
The right way is to visit MSM is to spend the night there. Arrive late afternoon - maybe 3 to 4 pm - so you can watch the human tide recede, it will make you feel smart for being there "after hours." Spend the night on the Mont itself if you can (there are a few small, quirky hotels on the Mont, they are not 5-star luxurious, they are a bit overpriced for what you get, but what you get is location, location, location..). These rooms get snapped up quickly, so if you want to sleep on the Mont, try to get them ASAP. If you can't/chose not to sleep on the Mont itself, then at least sleep nearby on the mainland. Plan your trip around the crowded times. Enjoy a few crowd-free hours late afternoon and early evening, sleep, rise at dawn, and get out and enjoy MSM in the early morning hours.
IMHO a rental car is the best way to tour this area - MSM is spectacular and special, but there are a lot of other wonderful things in the region - Normandy and Brittany. If you don't want to drive in Paris (highly stressful and to be avoided) you can train in and out of Paris, pick-up and return your rental car in a city on the way to Normandy.
MSM is a magical, dreamy, wonderful place. You deserve to see it at its best, without crushing crowds. The way to do that is to spend the night there. Wake up early morning after sleeping on the Mont, wander its empty streets, it will be the perfect travel experience you have been dreaming of. You will not be disappointed. Please consider doing this - it's worth the extra time, money and effort.
