I plan to spend few days in Paris in July with the following schedule:

9/7 (Sat)
City walk – Notre Dame, Musee du Louvre, Musee d’Orsay, Arc de Triomphe, Champs Elysees, Tour Eiffel

10/7 (Sun)
Day trip to Loire Valley, plan to visit Chenonceau, Amboise and Blois castles

11/7 (Mon)
Day trip to Mont. St Michel

12/7 (Tue)
Half day visit to Versailles; city walk along Champs Elysees

I travel with a 9 years old boy. We are not very interested in Museum. We browsed the internet and found that Mont. St Michel is a wonderful place to go. Due to time limitation, we can only spend one day there. Since long hour travelling is required, want to know if one day visit is good enough.
Tks.

If you are able to get to Mont St-Michel early in the day, before the crowds fill the streets and make for slow going, you can see the Mont's attractions in about four hours. When you add in travel time from the Loire Valley, that would be about right for a day trip, assuming you are staying the night in that vicinity.

I also plan to have a day-trip from Paris to Mont St-Michel in late July by train or public transport. How can be there? Thank you.

I noticed you are doing Loire and Mont St Michel back to back.

I have done Loire to Mont St Michel without having to go back to Paris. This allowed me to spend overnight at Mont St Michel. Staying overnight allows you to see the famous tide coming in, take a walk thru passages pretty much all by yourself, visit a cool night light show in the Abbey (same place, but different routing and different decoration from the day time, and with music.).

Thanks for the advice. Is train or bus available from Loire to Mont St Michel?

I would consider combining the Lire Valley with the Mont St. Michel. Drive in the LoireValley and then go toward the MSM and stop in Vitre or Fougeres. In my mind it makes more sense than going back to Paris.