So you are travelling to Switzerland from Rome, spending a day, and then continuing to Paris the next day?
I assume you already have the tickets Rome - Switzerland, and Switzerland - Paris?
If not, I would suggest that you stay in Luzern, and book your train travel from Rome to Luzern and Luzern to Paris in advance on www.sbb.ch. so you can get a discounted fare.
Then on the day you have for sightseeing just go to the Titlis (if that is what you want) buying the ticket for your whole family at the station in Luzern. One very short trips there is not much you can do to make things cheaper. Public transit in Switzerland is priced on purpose so that occasional use is expensive, but regular use is cheaper…