Welcome!

So first, you’ll need to buy your Paris travel locally, including trips to/ from Marne-la-Vallée (the station for Disney). It’s not included in any pass.

The Eurostar from Paris-Brussels requires a seat reservation, that costs €25 per person (even the child), if you’re using a pass. If you look on the Eurostar website, you will see there are regular tickets as low as €39 per adult, and €15 per child on that date, so it might be worth buying tickets instead of buying a pass - especially if you can decide on a train now.

So we are looking at a pass for Brussels-Cologne-Strasbourg-Switzerland.

A Eurail global pass would suit you for most of this. The child’s pass will be free when you buy an adult pass. You just have to decide how many days of traveling you think you will do in total. You can see the options here; https://www.eurail.com/en/eurail-passes/global-pass

This will cover all your travel, except the part between Interlaken and Grindwald. Eurail passholders can get a discount on that journey.

And you don’t need to pay any reservation fees as long as you 1) avoid the Eurostar from Brussels to Cologne (take the German ICE instead), and 2) don’t sit in the Panorama cars on the Goldenpass Line (the regular cars still have a very good view), and 3) avoid any French TGVs on the journeys to/ from Strasbourg.

I count 5 days, if I’m reading your itinerary correctly:

Brussels-Cologne
Cologne-Strasbourg
Strasbourg-Grindelwald
Grindelwald-Montreux via the Goldenpass
Montreux-Geneva Airport

So a 5 day in 1 month pass, would suit you. $239 per adult, and $0 for the child.

A 7 day pass is $282, and would give you a bit more flexibility if you want to do any extra travel.

Some advice :

Buy reservations on the ICE to Cologne, even though they're optional. That train is always very busy and you can get reservations for 9.80€ total for the whole family through DB.

I'd also get reservations on the Cologne - Karlsruhe/Offenburg ICE in case. No need to book long in advance.

Avoid the cross-border TGV/ICE into Strasbourg, it is unnecessarily expensive : 17€ seat reservation for a few km in France. Instead take the Offenburg - Appenweier - Strasbourg regional train.

You indeed get 25% discount in the Interlaken Ost - Lauterbrunnen/Grindelwald (and above) trains. Be aware of cable car maintenances in the off season -> First will be closed for example.

Reservations are optional on all Golden Pass trains. I've never made one and wouldn't in the future.

Interlaken - Grindelwald isn’t included in the Eurail Pass, but you do get a 25% discount.

This brings the price down from 11.20 CHF each way per adult, to 8.40.

You can buy the ticket at Interlaken or, as I discovered, from the conductor on board a Swiss train.

Some extra tips:

Brussels Cologne

The ICE Brussels - Cologne has very poor technical reliability lately. Delays and cancellations are possible. Luckily there’s a good alternative with regional trains (via Welkenraedt and Aachen to Cologne) but will take longer (no seat reservations needed). Don’t plan too tight or leave too late, but since this is your only connection that day it won’t be problematic.

Usually I suggest to avoid Eurostar (Red, ex-Thalys) here with a pass, as it’s reservations are too expensive.

But if you could score a cheap advance ticket for this connection with Eurostar (from 16 eur, average price is usually 30-40 EUR for an adult. You’re probably in for a much more reliable ride than with ICE. Ordinary tickets should still slightly be cheaper than an the cost of an average pass day. You might want to consider this.

Usually I say that seat reservations are not really necessary in low season for that ICE Brussels-Cologne (and often wasted because of the poor reliability atm). Head directly to carriage 22 in Brussels to find plenty of free, unreserved seats. But with a family, you might want the comfort of a seat reservation. Your choice.

We’re eagerly awaiting new (and hopefully much more reliable) trains somewhere next year.

For Paris Brussels

If Eurostar Red (Brussels-Paris high speed train) is too expensive both reservation wise and for ordinary tickets. You can always book seat reservations to Lille with TGV Inoui (plenty of availability) and continue your journey with reservation free regional trains to Brussels. This will go via Kortrijk/Courtrai (beware of the different city spelling in Dutch and French in this region) or via Tournai. Will only cost 10 EUR per seat reservation on the TGV.