One day in Switzerland--Is Jungfraujoch worth the price?

I think you can have a lovely day in Switzerland without spending that sort of money. In fact, I know you can, because I have spent far more than a day in Switzerland over the course of multiple trips and have never paid the big money to go to the top of a mountain!

Perhaps folks who have been to Switzerland recently (which I have not) can suggest a good but low-cost scenic day, keeping in mind that you have a Eurail Select Pass. They'll need to know where you are traveling from and where you are headed, so it may help a lot if you list your plans for the two or three days before and after your mountain day.

The problem with only having one day is that you are setting yourself up for disappointment due to weather. The mountains make their own weather and it can vary quite a lot during the day. On even a clear and warm day cloud can bubble up in late morning and if you are up the mountain you risk looking out into fog.

It is far far better if you have at least two days so that you can be early in the day, review the cameras on your hotel TV and decide which walk or activity you will do that day.

What time of year is this?

I'm really confused by your time scale. You say you have only one day, yet you talk about making a trip from Interlaken (means between the lakes, is at lake level and is NOT in the mountains) to Lauterbrunnen. Does that mean that you are in a hotel in Interlaken? If so I strongly recommend taking a hotel in one of the villages actually in the mountains. Then you say you are going to Luzern. Does that mean you are staying in Luzern?

Where in your travel plan are you trying to shoehorn Switzerland in? Where are you arriving from, where are you going to?

Or are you just flying over for one day or a weekend and flying back?

We have a bit of a whirlwind tour and are simply passing through. Our schedule is packed and more rigid than I would like really, so extra days are not possible, unfortunately. We are flying into Lisbon, and after a 14 hour layover to explore the city, flying into Milan (Just because it's cheaper). We sleep in Milan, then head to Rome for 3 days. After that, we have 2 days in Florence. We will leave Florence on the 2nd evening and sleep in Switzerland. We have the whole day for a scenic ride, then we will be simply passing through Lucerne on our way to Paris, where we will sleep that night. We have 4 days in Paris, then take the Eurostar for 3 days in London. The reason for the Milan to Rome to Florence route is due to interference by museum closings and the like. We will be going in April. We could skip Switzerland entirely, take an extra night in Rome, but I haven't had a trip like this in over 20 years (not sure when the next one will be) and I really wanted to 1) see something very different (particularly the mountains) and 2) have a low key day to just enjoy the view from a train.

I'm still a bit confused. Do I have this right?

On your second day in Florence you head in the evening to Switzerland. The 5 PM train from Florence to Lauterbrunnen arrives at 11:23 PM. I don't see any later trains (using the Deutsche Bahn website) that get you to that part of Switzerland the same day. Where are you planning to spend the night?

Then you indicate that you have all day for a scenic ride before heading through Lucerne to Paris. Do you mean that you will spend your scenic-ride night in Paris, or are you targeting Paris for the following night? I hope the latter, because the last train from Lucerne that reaches Paris the same day departs Lucerne at 5:30 PM, so if you expect to reach Paris the same night, you'll need to truncate your scenic ride to be sure you are in Lucerne in time to make that train. It takes nearly 2-1/2 hours to get from Lauterbrunnen to Lucerne, just as one example.

Skip Switzerland and add a day elsewhere. Visit Switzerland some other time.

Sounds absolutely grueling, and with a 5 yo in tow.
I am guessing you already have your airline tickets. Otherwise, why fly to Milan, which you apparently have no desire to see, just to turn around and take a train to Rome the next day, where you really want to be, trying to save money. One of Rick's mantras, spend money to save valuable vacation time.
I am also guessing that you already spent a gob of money on Eurail Select Passes, so now you are committed to traveling by train to get to Paris.
According to www.sbb.ch/en, the full fare cost of 2 adults and one 5 yo (traveling at half fare), traveling from Interlaken Ost to Jungfraujoch and returning via Grindelwald, will be 527 CHF. Your Eurail Pass only gives you a 25% discount for this, so that is about 395 CHF, basically equal to USD at this point. This is the single worst value using a Eurail Pass.
So here is my modest proposal to get to Paris, still see some mountain scenery (if the weather cooperates).
Travel from Florence to Kandersteg. Leave at 3 pm and you can get there before sundown (8:15 thereabouts).
It is a scenic ride up "the ramp" from Brig to the old Loetschberg tunnel. Kandersteg is at the exit from the tunnel and it is in the mountains. Next morning, check the weather, and if it looks good, travel to Basel via Interlaken and Luzern to do the scenic ride over the Bruenig Pass. If the weather is not good, sleep in and take a later train to Basel via Bern, a lot more direct. You will need to reserve as soon as possible, the TGV from Basel to Paris, for 4:34 pm. That will get you to Paris before sundown. You need to reserve ASAP the TGV train, as they may limit the number of Eurail Pass holders allowed on the train, no matter how empty it otherwise is.

The altitude is actually 11,371 feet. I see 5 yo's skiing in Colorado at that elevation all the time, and given your killer schedule, you'll only be there for an hour at most if you do go.

My simple answer to your question is: based on your itinerary, no.

If traveling to Paris from the Lauterbrunnen area, the best train connection is normally through Basel, not Luzern. Most days, the last direct train from Basel to Paris departs around 18:30, so you'd have to leave Interlaken Ost station by 16:00 (I would not choose a later departure with a 5 y.o.).

In my visits, I've prioritized the view from the top of Mt. Schilthorn, which is faster, less expensive, and gives you a great 360-degree view. If you go up the other side of the valley, then I'd just go as far as Kleine Scheidegg and walk around there. Your pass gives you 25% off any of these connections from Interlaken Ost up the valley and mountains.

I wouldn't go up to the Jungfraujoch because of time involved and from what I've researched a lot if the trip is actually inside the mountain where you can't see much. We're booked to be in Wengen for four nights this July and feel our time will be better spent outside. A couple of friends in our group are planning to ride the train up but they have 7 nights. You can ride up to the Schlithorn peak on a cable car - but again you have limited time.

Ok, thanks all, for the feedback. It is admittedly a vigorous schedule, but that tends to be how we travel. (We're funny that way.) We will still do our traveling through Switzerland, as I think it will be more enjoyable for me (even if taking a full day) than through France. I think we will skip the Jungfrau itself, and opt instead, to just make the rail loop from Interlaken, Laterbrunnen/Kleine Scheidegg and then back through Grindelwald. An acquaintance in Switzerland has timed the trains out, so that should be fine. As I said, the purpose of the day is to sit back and enjoy the scenery from a train for the most part, but for now, we'll opt out of the added Jungfrau expense. I appreciate the advice.

If the weather is nice the valley walk itself is very scenic.

If you want some pictures just pm me your email. Or google it.