Jungfraujoch Train - discounts available?

If you take the first train or so in the morning, there is a "good morning special", with about a 10% discount. Otherwise, get a Swiss Half Fare Card for 120 chf and you can ride at 50%. You can always wash up at the Kleine Scheidegg station before heading up for the final leg. You have to change trains there anyway.

I recommend buying the tickets after you arrive and you know you have clear weather.

Depending on what other touring you plan to do in that area, one of the Jungfrau Passes may work for you. It's not exactly a "discount" but if you use it a lot for the three days it can be somewhat cost effective. As mentioned in an earlier post, it's important to choose which day to visit, as if the weather is bad you won't see much. One of the other Swiss passes may be a good fit also, but you'll have to do some number crunching based on the touring you plan to do.

I don't recall any washrooms on the Jungfraujoch trains (they're not that big). The trip isn't long anyway, and there will be washrooms available at Kleine Scheidegg prior to boarding the Jungfraubahn.

and plenty of them at the summit.

The best deal for your own children (or grandchildren) under age 16 will be a Junior Card for 30 francs per child. That investment allows them to ride free on any public transport where the adults buy a ticket, including this train and other lifts. I'm sure the ticket agent would suggest this option.