More on the Travelcards and 2-4-1 offers:
We have used these many times to get the 2-4-1 deals.You need a "paper travelcard" purchased from a National,Rail Station, NOT a Tube station. You can buy at Paddington, Waterloo, London Bridge, Kings Cross, and others, but make sure younare at the rail station window, not the Tube station window. The card will cover unlimited travel by Tube and bus for 7 calendar days, and can give you a discount on a boat ride on the Thames waterbus ( not the cruise boats).
The cost for adults is £35.10 for a Zone1-2 Travelcard (what most visitors will need). The 17-year old counts as an adult in this system. The 15-year olds are considered children ( under 16 if the usual rule) and their Travelcards will cost £17.60.
You can then pair up, with the two adults using one 2-4-1 offer, and the two younger children using another. Younwill need two vouchers for each attraction and it is best to print them in advance, as the stations often run out of the little voucher booklets.
The 17-year old will qualify for a "student" concession price at some attractions, but now all.
Using the 2-4-1 offer at the London Eye, for example, will save you one adult and one child admission, so £30 plus £24. Note that you can get 10% off those prices by purchasing in advance on the website, but then you cannot use the 2-4-1. And to get "fast track" or skip the line, you would have to pay an extra £10 each, and still not use the 2-4-1.
At Westminster Abbey you can save one adult and one child admission, for a total of £ 29 in savings.
Churchill War Rooms have timed entry tickets and it is best to buy those in advance, without the 2-4-1 offer.
Tower of London has a very complicated price structure, with family prices which may benefit you, so check that website carefully.
Many of the great museums are free, it they may have special exhibits which domrequire tickets, and you can usually use 2-4-1 on those.
Just be sure to get the right kind of Travelcard:
https://www.daysoutguide.co.uk/travel-by-train/is-my-ticket-valid-for-2for1