7 nights in Switzerland (coming from Paris)

5 and 2 sounds fine to me.

You are traveling to a remote (transport wise) location, so you have to expect it will take a few train changes to get there. You should take a TGV from Paris to Basel, then regular trains the rest of the way. Depending on what you are planning on in Wengen, you might want to buy Half Fare Cards for 120 CHF per person. Then all transport inside Switzerland will be 50% off list price.

Swiss figure people don't want to hang around in train stations, so they have tight connections, which are makeable. But if you do miss a connection, your ticket is good on the next train, and there is always a train every hour in each direction, more often every half hour. Going Basel to Wengen, you can get a direct train to Interlaken Ost, but if there is a connection (in Bern), you may decide to take it if it gets you there sooner. At Interlaken Ost, you change to a train to Lauterbrunnen, and at Lauterbruunen change to a train to Wengen. The last two trains are narrow gauge and use cog-and-rack systems to climb steep grades.

is the trip from Paris to Wengen going to be a lot for one day, by train?

You would book this part first which is Paris to Interlaken OST (choose the one where you change trains once).

Then at interlaken Ost buy the ticket to Wengen out the machine. It will require a change in Lauterbrunnen and thats it.

Does Wengen for 5 nights and Lucerne for 2 sound like a good itinerary?

That's a good balance. I think 7 is too many not unless you are writing a book and want to get away from it all.

Thank you so much, this is extremely helpful!

Does Wengen for 5 nights and Lucerne for 2 sound like a good itinerary? Should we just do all 7 in Wengen?

Since you only have 7 nights for Switzerland, then I think you have a good itinerary with 5 nights in Wengen and 2 in Luzern.
As far as I'm concerned, I can never get enough time in the Wengen area, and I've spent a month several times, taking day trips in that general area. And since it's not unusual to have foggy or rainy weather there, there should be an allowance made for extra days to experience the best hikes--all different.
However, Luzern is not to be missed--I always enjoy spending some time there, too.

I've done the Paris to Wengen trip and I know how long it takes.

In Paris, you have to get to the train station Gare de Lyon (30 min?)
Then assuming you have reservations for the bullet train, that takes 3 hours to Geneva. (Also trains to Basel/Zurich.)
You should buy these in advance and you can find loads of info on that at www.seat61.com
I seem to recall the Paris to Geneva train goes 9am to noon.

You may or may not get tickets for Switz. in advance (sbb.ch/en) but they can be easily bought at the Geneva station.
From there you can get tickets all the way to Wengen. (Change trains at Bern, then Interlaken Ost , look for "Lauterbrunnen" on the side of the train car), then finally at Lauterbrunnen. That's another 4 hours.

So that's 7 hours on trains, but they are comfy and smooth and the layovers are short.
I've done this trip many times from Paris and NEVER had a problem because you can set your watch with the Swiss trains.
The ticket agents in Geneva are pros, and can give you a printout of all the connections, and track numbers, etc.
Rick Steves has some good videos on this, but worst case you go up to any conductor and ask "Lauterbrunnen?" or other name. They all speak English. If you miss a train in Switz., there will be another 30 min. later that you can take with the same ticket. Just don't miss the reserved bullet train!