Hi there, please peek at my itinerary?

I'm not sure but $5000 is a lot of money. I wouldn't mind back tracking for that amount.

Do we stay in Switzerland for a 2nd week and accept the additional $5000 flight fare to fly home from there?

Presumably that's for 5 (?): what cabin class has that big a difference?

Are you booking one-way tickets or multi-city ( aka open jaw) flights?

The savings are significant. If you do end in Paris, I'd stay on the train. It's not so silly to not stop there first if you are going back there anyway at the end.

Why not stay in Beaune. France on the return train ride to Paris? Or Strasbourg? They’re on the way and that gets you out of repeating a big city. Train tickets from Interlaaken to Beaune and then to Paris can be a bargain when bought well in advance.
So, you’d also save a truckload of euros over flying home from Switzerland.

How set are your accommodations? If they are not changeable that will make my thoughts unhelpful, but I'll share anyway.

I myself would move Paris last if the airfares home are $5000 cheaper from Paris. Just know CDG is a real pain and you'll want to give yourself PLENTY of time there on departure day. When I left in August on a weekday afternoon it took 50 minutes from check in to the gate.

Is Europa Park a deal breaker? It seems like an oddly placed stop with not great logistics to get there. There are train options from Belgium to Germany that don't go through Paris and they take the same amount of time. All options have a lot of train changes and will be a VERY long day, so you might consider breaking it up (just not in Paris). A well placed inter-Europe flight might save time and money (eg., Brussels to Zurich) but you would have to research.

but the train from Brussels to Europa Park area passes right through Paris anyway

No, not necessarily. If you want to make an international train journey, departing in Belgium, my advice would be to use the (internationa) planner of the Belgian National Railway company https://www.b-europe.com/EN
From Brussels-Midi to Ringsheim they will suggest an itinerary through Germany (without entering France), i.e. via Frankfurt and Offenburg.
Perhaps this situation could facilitate your decisions.

Oh yes I forgot that from Paris to Europa Park, we were going to stop in Strasbourg. Forgot to add that.

Yes $5k is significant! I'm wondering how accurate google maps is when it estimates time on public transit? It says Interlaken to Paris would take about 5-6 hours on train, with multiple transfers. I'm thinking by the end of a 6 week trip, we will be very tired so I'm trying to make our last stop or two as seamless as possible. Is it possible it would not take this long in real time?

My accommodations are all cancellable if needed. I had an idea of going from Belgium through Germany and stopping at Stuttgart for the car museums there (for my car loving son), but I think Europa Park would appeal more to all 3 of my kids.

Is there a better order I should consider?

Also, I thought open jaw was the same thing as 2 one-way tickets? Is it not? We are planning to fly into London and home from...well, that's what I'm trying to figure out. Are two separate one-way tickets not the way to go?

Autoworld Brussels could offer an alternative https://www.autoworld.be/en/

Also, I thought open jaw was the same thing as 2 one-way tickets?

No, Open Jaw (often called Multi City) is where you buy a return ticket, flying into Place A, flying back from Place B. It should usually be way cheaper than two one way tickets.
As an example, if I wanted to fly into New York from London, go overland to Seattle (or fly there several days later on a domestic ticket), then fly back to London from Seattle, I would buy an Open Jaw/Multi City ticket, not two singles.

If you look at either the SBB (Swiss Railways) or SNCF (French Railways) website for tomorrow you will see that the Interlaken to Paris journey indeed takes about 6 hours, but can be done with one change of train- at Basel- for instance 0959 from Interlaken Ost (or 1004 from Interlaken West) Basel SBB arrive 1201 depart 1234 Paris Gare de Lyon arrive 1555

Also, I thought open jaw was the same thing as 2 one-way tickets? Is
it not? We are planning to fly into London and home from...well,
that's what I'm trying to figure out. Are two separate one-way tickets
not the way to go?

As the prior post explains, an open jaw/multi city flight search is what you want to use. On Google Flights (flights.google.com) you use the pull down menu on the search screen to select "multi city" (instead of round trip or one way). Put in your home airport, destination, dates, and put in the two different cities you want to start and return from. I picked a random six week period in July/August 2026 and searched JFK-LHR and ZRH-JFK and for five people it was under $4k for all five tickets (direct flight to London, stopping in Madrid on way back). YMMV, your dates could make the prices different (cost varies by day of week/holidays/etc).

PS don't forget you'll need an ETA for each of you to enter the UK. Plenty on that topic in the forum, you don't need that info to book a plane fare.