Firstly, are you saying that you are only going to make 4 trips on public transport?
1. St Pancras to Pimlico
2. Pimlico to Watford Junction
3. Watford Junction to Pimlico
4. Pimlico to Gatwick
If so, there is no need to get a travelcard - just pay for each individual trip.
All these trips can be done on Oyster, but for Pimlico to Gatwick it may be cheaper to buy rail tickets because of the children. Do not buy Oysters beforehand unless you like wasting money.
If there is going to be no other travel on public transport that day, and you can commit to a specific train time - then book tickets online on Southern Railway from Victoria to Gatwick. This will be £12 for the adults and £2 for the children, however these tickets can sell out and can't be bought just before travel. You must take the train specified on your ticket.
Walk from Pimlico to Victoria or take a bus (free for 9-year-old, 11-year-old see below).
If there are no £12 tickets left the cheapest option is £35.15 in 2019 which will rise to £36.05 in 2020 for all 4 of you but the 11-year-old gets an adult ticket as it is cheaper. With these tickets you can take any train.
If there is other travel that day then you need to specify, as the options will differ.
For the remaining travel, the adults can use a contactless bank card (one each) provided that you have no high fixed fee per foreign transaction. Otherwise they can buy Oysters at Kings Cross St Pancras underground station.
The 11-year-old can get half price travel by asking a member of staff to set the Young Visitor Discount on their Oyster. Or if you are only going making these three trips and you have a third contactless bank card, just pay adult fare and don't worry about the extra cost.
The 9-year-old travels free when accompanied by an adult.
I'm not sure if you can even get a paper travelcard to Watford Junction, which is not in Zone 9 by the way.
Last edited by :D!; Dec 15, 2019 at 1:59 am