It looks like you'll pay CHF 150 (about $150). Swiss Pass should pay all up to Wengen, then from Wengen to Kleine Scheidegg to Jungfraujoch and return is 25% off the price. Kids free with junior card.
Here's more info:
Bearers of a Generalabonnement SBB or a Swiss Travel Pass may travel for free as high as Wengen, Mürren and Grindelwald but pay half price for further trains. Swiss Travel Pass holders receive only 25% discounts on the railway sections Wengen-Kleine Scheidegg-Jungfraujoch-Kleine Scheidegg-Grindelwald.
"Children under age 6 travel free. Children from age 6 to 15 with a Junior Card also travel free to the Jungfraujoch, if accompanied by at least one parent . The Junior Card is available at all railway stations for CHF 30.- and is valid for one year. Without a Junior Card, children age 6 to 15 pay half fare."
This last website has loads of information that you may find useful. Having said all that, I would NOT advise getting your tickets now. The day you pick may be clouded up, or even a cloud covering the mountain top but everything else is clear. I would buy tickets the morning of, or evening before, if you know the weather forecast. Also, earlier is better as mountains tend to cloud up later in the day.
Also, think about alternatives. The trip to the top of the Schilthorn is about half that price and some people like it better because you aren't that far away from everything and you can actually make out details below. Or, since I've taken teens there many times in the past, most are perfectly happy exploring the trails and petting cows and goats along the way. If you want a good overview of those trails, click on my name and you'll find a link to our dozen favorite trails in the area, with maps and pictures.