Yes Veronica, you can do a bus trip from Paris to Loire Valley as a daytrip.
I have.
There are two very well known and long established companies that both do big bus tours and mini van tours. They are expensive,about 150 for the big bus tour and more for the mini van tour, but that includes transport and admissions.
I used Cityrama, but Parisvisions does exact same tours too and cost is similar( I think they were a few euros more so I just chose cheapest) .
You can also use a company called Acco Dispo , but they do not leave from Paris, they leave from Amboise and you are responsible for getting train there yourself.
I have done the Loire Valley twice. Once with my father and we rented a car and visited about 6 or 7 places in three days, very nice.
On the second visit it was just my 14 yr old son and I and I did not want to rent or drive a car, so we took the bus tour. We saw three chateau. It is a long day, but we enjoyed it. The guides were good, I liked the way they got us in with no lining up, gave short tour and speech ( usaully about 45 minutes) and then gave us another hour or so on our own, and then had us meet them at the bus.
The worst part is you leave Paris so early in the morning there was no where to have breakfast , but I did buy some juice and cookies for the morning bus ride .