I think I understand the difference between a 7-day Travelcard and an Oyster card, and it sounds like the former (Zone 1-3) is the best option for us. The only part that isn't quite clear is rail service. One day we're taking the train to Salisbury. I suppose we take the tube to the nearest light rail station (Clapham Junction in our case) then buy a rail ticket to Salisbury? There's no benefit of using Oyster Card or Travelcard with rail?

We're in London a total of 8 days. If I put Ł10 on each card the first day, can I apply what is left over on the card toward the roughly Ł48 for a Zone1-3 7-day card? For instance, if each card uses Ł6 the first day, can we put the remaining Ł4 toward the Travelcard so it's Ł44 each?
(I know folks are very fond of contactless credit card payment, yet my teens don't have credit cards.)

Thank you!

In your situation, a rail 'travel card' isn't really useful for much except for the 2for1 discounts on the covered day(s). I personally wouldn't bother.

I'd make things as simple as possible. I would use OysterCards each loaded with a 7-day zone 1-2 travel card plus a few extra Ł for use on the 8th day, and book regular train tickets for your day trip to Salisbury. (BTW there is no 'light rail' involved with a journey to Salisbury - just take a direct train from Waterloo Station)

Excellent - thank you so much!

What age are your teens? if between 11 and 15 you can get them Oyster cards and ask for the Young Visitor Discount to be added to them. they will then get half price travel. If 16 are over they are classed as adults for travel purposes.

Whilst they might not have credit cards they may well have phones? If so you could just add your card on to their phones and they can use them rather than Oysters if they are too old for the discount.

One is 15 - definitely getting the young visitor discount! They have phones but I've been unable to get the TFL Oyster app to work on my phone from the US. I've read here that it's not worth trying unless in the UK, and don't work to mess with getting everyone set once we're there.
Not comfortable adding my credit card to their phones. I've recently had an issue with my phone trying to pay for other people if it's placed too close! Maybe that's just a US thing, or perhaps it's just in our community that the contactless payment readers are overly sensitive.

There is no point in trying to get Oyster cards in advance of your trip. The Visitor ones cost more as they have to be posted out and are less flexible. There is also no need to get the app either. Just plan to get the Oysters when you arrive. Buy the cards and ask a member of staff to put the Young Visitor Discount on the one for your 15 year old.
You should not be able to get your phone close to another when clocking in and out of the TFL system. But if you're uncomfortable don't do it. All it will mean is paying an extra Ł7 for an Oyster for your other child.

The Visitor Travel card is of no benefit really - Oysters are soooooo easy to buy in any tube station.

If you have a contactless bank card, you can use that instead of an Oyster card for transport within London (but for teenagers' Young Person's Discount the Oyster is unavoidable).

London to Salisbury is a National Rail service, so yes, it's ticketed separately. See www.nationalrail.co.uk for times and fares - it will link you the operator (South West Trains) to buy them.

Thank you for the train advice and the link, though South West Trains will not allow me to order tickets from the US. Very disappointed! Just spent an hour trying to navigate the system and understand the different types of tickets. Not sure where to go now, probably best to simply buy at the station despite the increased cost.

When is the trip?? Currently tickets are only available up to 19 June.

Try using thetrainline.com for tickets. They add on a small fee but I find the website easy to navigate. I live in Australia and have bought train tickets for UK and France. Website should work in any country I guess. Tickets can be emailed, then you can choose to scan at the station from your phone or print before you leave home.

I buy train tickets from out of country a fair bit and normally the 3 months rule is the first thing that gets in the way, international sales should not be a problem, unless you want a paper copy. Either take an eticket or choose the station you want to print off and just physically get it when you are there.

seat61.com is a great source of info for all of this

You can use any train operating company in the UK to buy your tickets from - it doesn't have to be the actual operator of the train you want. I'm not aware of any issues with SW Trains and overseas cards but that could be an issue - it is a well known issue with GWR. Northern Trains is usually reliable in this situation.

I would not use The Trainline - they charge a fee for bookings, albeit only a small one. But using them means putting another company between you and the company running the trains, so any issues on the day are made more complicated to sort out.

I had thought it might be too far ahead for train tix . . . but did a little sleuthing and it seems they are traveling THIS week. On another thread the OP talked about being in London over Easter weekend. If this is the case -- rather that too early, they are are quite possibly too late and the trains are booked up???

We need some clarification . . .

The trAins won’t be booked up. There doesn’t seem to be any issue with booking - I just looked for tomorrow. But it’s too late for the cheaper tickets.for this weekend.

Just a quick follow up - I tried multiple times on computer and phone to make train reservations online before departing the US. The Southwest Trains website simply wouldn't allow the purchase six days prior to departure. The error message stated that the address must be a UK address. In retrospect perhaps I could have attempted using the address of the place we were staying in London, but didn't want that to backfire with the credit card...

I tried to purchase the tix at the train station kiosk three days before travel and the price was about Ł30 more per ticket than online! So I went to the ticket agent to see what the issue might be. She was able to get the price down to only Ł15 more per ticket - not as great as the online option (plus online it was an anytime ticket vs specific departure times) but better than nothing. She also didn't seem to know why I couldn't purchase online, but it was Easter Sunday and I think she would have preferred not to be working, so I didn't press it or complain!

I haven't used that particular website for tickets but you shouldn't need to enter an address. Were you hoping for tickets to be posted to you? Train tickets generally are similar to airline tickets, more expensive the closer you get to departure.

Nope, just emailed. I tried selecting the option to print at a station kiosk but that didn't work either.
The site definitely required an address. I even tried not entering one, but got an error message that address is required.
Understand they'd go up in price, which is why I tried a week ahead of time, then again six days ahead. Really didn't want to wait until arrival but nothing I could do about that.

As I said in post 13 you could have used any one of the other train operating companies’ websites, as they all sell each others’ tickets.

Ditto ^^^

Plus just a future FYI -- the few sites that do require a UK address for something, they don't ask for it during payment/credit card verification. Just use your hotel's address.