I will be traveling with two daughters in September from Germany to Paris, to Basil, Switzerland to Interlaken, Switzerland, and then by air from Interlaken or connecting? to Rome. what are the best priced your rail passes or tickets, and for the one time flight from Switzerland to Rome?
From where in Germany are you traveling to Paris?
From Interlaken, you will need to go to either Geneva, Zurich, or back to Basel to get to an airport that flies direct to Rome.
As far as rail passes, there are Eurail Global passes for 4 days of your choice. Assuming your daughters are under 26 (at the time of travel) they will get a discounted Youth Pass. French trains require the purchase of seat reservations. Eurail is showing $661 for 1 Adult and 2 Youth for 4 travel days at current sale price.
However, if you can fix your travel days, you can buy train tickets now at discounts that will cost less than rail passes. Seat reservations will be included in the price.
From the phrasing of the question, I suspect the OP thinks they are speaking directly to a Rick Steves staff member. This forum is peer-to-peer support, so while posters here are extremely well-informed and well-traveled we are not RS staffers.
However, DO follow the advice above from Sam. You should price this out as a rail pass may not be the best way for you to go.
If this is a trip planned for next year or later, there is also the option of the overnight train from Switzerland to Rome.
And not just an overnight train. Right now, it takes 9 hours or less to take a day train from Interlaken to Rome, with several changes along the way. It will take at least 2 1/2 hours to get to one of the airports from Interlaken, check in 1 1/2 hours navigating the airport, going through security, be at the gate well before flight time, 1 1/2 hours in the air, 1 hour from the plane landing in Rome and you are actually in Rome city center. That makes it 6 1/2 hours to fly.
Not only are we not Rick Steves, but Rick Steves is not a travel agent, so he and his company do not book rail or flights. You can of course use an agent to do this, but they no longer book tickets for free. You can do it yourself online with the following advice.
To find train prices for trains starting in Germany, as well as rail schedules for all of Europe, use the Bahn (German Rail) website by following Rick's tutorial: https://www.ricksteves.com/travel-tips/transportation/trains/online-schedules
To find rail prices within Switzerland, use Swiss Rail. https://www.sbb.ch/en/home.html
Note that the first price shown is with a Half Fare Card. To find the full price, be sure to keep clicking through and choose "no reduction."
The Man in Seat 61 is a rail guru. His website has everything you need to know about trains, and spending some time on his website is a very good idea if you've never taken trains before. Start with his general introduction to European trains: https://www.seat61.com/european-train-travel.htm
and his discussion of rail passes: https://www.seat61.com/how-to-use-a-eurail-pass.htm#railpass-or-point-to-point-tickets
To find flights within Europe, use Skyscanner: https://www.skyscanner.com/. Be sure to check all airport options. In Switzerland, try Zurich, Geneva, and Basel; for Rome, make sure you're looking at Rome Fuimicino (FCO) and Rome Ciampino (CIA).
Flights within Europe will have all kinds of strict rules, all kinds of fees, and very high fees for "breaking" the rules. For instance, if they say only one carry-on is included in your ticket, they mean one bag - not one bag plus a purse. You can pay more to get more luggage at the time of ticket booking - or you can pay a LOT more if you wait until you get to the airport and they see that you have more luggage than you previously paid for. Similarly, some airlines require that you have printed out the boarding pass, or have it on your phone via the airline's app, BEFORE you get to the airport, and charge a lot of money to print one for you at the airport. So, be sure to do a dummy booking (up to the point where you put in your credit card information) to see ALL of the fees and rules.