Hi Memorylane -
Arno asked "When."
It matters in the Alps, because Winter lingers at high altitudes.
That is a separate issue from when you need to win the lottery.
I'm not sure how you got £500.00, but trips to the Jungfrau alone ( which you mentioned) would certainly add up something near that, by themselves. Not to mention the cost of Swiss Travel Passes. So, I'd take the Jungfraujoch off the list, no matter what.
It probably still would be a bit much for you, but half fare cards for the adults at about 120 CHF each might be worth a re-check. Don't know about the 16 year old. Arno knows a lot more about passes that I do, so he has the details of the low cost add-ons for the younger members of your family.
And, of course, any of the discounted rail cards only pay for themselves if you make a lot of trips.
A peculiarity of the transport system in the Jungfrau region, due to the Swiss Federation policy to provide low cost transportation between population centers, is that fares from Lauterbrunnen to/from Wengen are very cheap. From there on up, the same cograil train continuing to Kleine Scheidegg and Grindelwald is then officially an excursion train with fares based on "specialty pricing" for the people who will pay for the views.
So, you certainly could go up to Wengen, as well as stay on the valley floor. There are some nice views from Wengen. Also, as noted above, c\heck out the cost to ride up to Grütschalp from Lauterbrunnen,, walk to Mürren, take the cableway down to Stechelberg ( two stages,; you could walk the first stage as far as Gimmelwald) and walk or bus back to Lauterbrunnen. Apart from your campground, you can park at the station in Lauterbrunnen...a bit pricey...or Stechelberg.
I don't know the costs to park at Stechelberg.
If you go up to Wengen , and are there after mid to late June (when the trail opens) , give some though to making your chosen cableway venture from Wengen to Männlichen, and then walk to Kleine Scheidegg (easy).
It is easy enough to have the nickname "The Grannies Walk, and the views are spectacular. See attached. The young lady in the pictures is my 8-year old granddaughter in 2009.
Then, walk back down to Wengen instead of using the train.
That walk is not difficult, but can be hard on the knees because of the elevation change...downhill.
Not sure how your 5 year old would handle that.distance.
If you read topo maps, this one will show the trails and the altitudes:
map.geo.admin.ch/?topic=ech&lang=en&bgLa yer=ch.swisstopo.pixel karte-farbe&layers=ch.bav.ha ltestellen-oev,ch.swisstopo.swiss tlm3d-wanderwege&layers_visi bility=false,true&E=26 37650.00&N=1161120.00& zoom=6
The color coding of the trails - simplistically - is that yellow ones can be done with no special gear, but you ought to have sturdy shoes or your boots, red ones demand boots and hiking staffs, blue are technical climbing.
There also some nice walks along the lake shore, near Thun and Faulensee.