We did the Overlord ½ day tour earlier this month and it was excellent - just the right amount of time for us. Your kids should enjoy it.
I agree with the others that you should take the earliest train possible but for an additional reason. Trains in France are at times affected by unforeseen delays, strikes, track work, etc.. You don’t want to miss your tour because of a train issue. Someone else who knows more about French trains can chime in, but here are my recent experiences.
We were in Bayeux for two nights earlier this month and our train back to Paris was canceled due to a strike. We were able to book a later train but we were not able to make our afternoon ticket time for the Picasso Museum. (We emailed the museum and they kindly reissued our tickets for the next day and luckily we were free to accept.)
On another trip, last year, we were notified two days before our travel was scheduled that all trains from Barcelona to Paris were canceled that day for a strike. We ended up flying. On that same trip we needed to cancel our Loire chateaux day trip as the trains from Paris to Tours were not running due to track work.