Train passes

Do not assume that a rail pass is an economical option. Historical rail passes were no-brainer good deal but with the promotion of advanced discount internet sales, seat reservation fees, limited seats for pass holders, etc. you need to do you homework to determine if a pass is appropriate. A pass does provide some flexibility but you will need to do your homework. Established your schedule and then check the prices for individual trips and compare to the price of the pass.

Buy advance Sparpreis Europa tickets for Paris to Munich and Munich to Lauterbrunnen when they become available for purchase, plus Half Fare Cards for Switzerland.

Sparpreis Europa tickets are changeable with a small fee, and at least in Munich, include local transport tickets.

Half Fare Cards in Switzerland are generally a better deal than Swiss Travel Passes if you have more than 240 CHF of full fare tickets per person.

You only have two longer journeys. Bought in advance Paris to Munich can be as cheap as EUR 49,90 (as an example: bought now for a date mid-September on Bahn.de, the German railways) and Munich to Lauterbrunnen EUR 31,90 (dito). Trains around Munich (including Salzburg across the border) are cheap with the BayernTicket: a whole day on regional trains at EUR 46 for your whole family.
I have no experience with Switzerland, but a rail pass might be a sensible choice.

We plan on using the train system for all travel and not quite sure what passes we need

First thing you must understand is that there is not one train system, there are (literally) hundreds of different rail companies; same as there is not "one air system", there are different airlines. The difference is rail companies rarely compete on the same route, they have different territories. There is usually one big national Rail company (SNCF in France, DB in Germany, SBB in Switzerland), plus many local companies.
And different countries have different ticketing and pricing systems.

As the others say, you only have 2 long trips. These are better bought as advance purchase tickets.

• Paris to Munich. This is a long way, it would be quicker, and probably cheaper to fly.

• Munich to Lauterbrunnen. The first leg (Munich to Zürich) is currently being re-built and most trains have been replaced by buses. There is an alternative routing via Karlsruhe and Basel which takes a bit longer (7½ hours instead of 6½), but is train all the way. Make sure you are aware which route you buy.

For Switzerland, A "Swiss Pass", or a local "Berner Oberland Pass" is probably the best option.
And in most cases it is cheaper to buy tickets direct from the rail company, not a reseller.
I suggest you read this webpage: https://www.seat61.com/Europe-train-travel.htm

Finally, Paris - Munich- Switzerland is doubling back on yourself. Consider Paris - Switzerland - Munich. Less travelling.

As suggested, I would consider Paris-Zurich-Munich and fly home from Munich.

As far as travel time goes, it is 5 1/2 hours each for Paris to Lauterbrunnen or Paris to Munich. Paris to Munich can be done with a direct TGV, or with a single connection in Stuttgart. Paris to Lauterbrunnen will be at least 2 connections. You are also likely to get a better price for the Paris-Munich train(s). So I would not necessarily alter your plan based on distances on a map. Price and travel time are what count.

Needless to say, Lauterbrunnen-Munich is the same price/travel time either direction.