Most economic Travel from Gatwick to London/ in London with 5 kids

Over the years' we have taken 5 grandson's to Europe and urge you to make the first day a slooow day. After working through Passport Control, getting baggage, and getting to the city, not only will you be a mess, but so will everyone else.
However, you have a real bonus-you have 2 teenagers who you need to get involved with the planning-they will amaze you at what they can do with a phone while you are traveling; city transportation, where to eat, how to get there. Finally, I can't remember if it was the NYT or WP that had an story or letter from a Grandmother who wrote something about how teens' made her travel sso much easier, and from personal experience, we took to to NYC after their miserable Covid experience and we were on auto-pilot as their phones guided u through the city.

I haven’t dealt with a group your size, but I have traveled from Gatwick to St Pancras Station. (I assume you are staying near there for convenience to leave on Eurostar the next day). I took the Thameslink railway from the airport and it could not have been easier. The airport is well signed and it will take you to St Pancras without any changes required.

Remember, the person most impacted by jet lag will limit your pace. Be flexible about what you do on arrival day. I would probably go to hotel, see how people were feeling, and walk over to the British Library.

All if your kids can get either free or discounted travel on the Underground but they all have to get photocards:

https://tfl.gov.uk/fares/free-and-discounted-travelBBK

As for the other info.....where are you staying and how much luggage do you have?

No the youngest children don't travel free on main line trains. After their 5th birthday they pay children's fare. And the oldest counts as an adult.

But on TfL services, when you are in London, the two youngest do travel for free.

The cheapest way into and out of London (hoping you are staying in the St Pancras/Kings Cross area) is to buy a family and friends railcard for £30 (as a digital product) and travel on a Thameslink train, off peak, defined as after 0930 on this route. Buy your tickets as a group on the machines on arrival.

Without a railcard you will pay as a group £54.40 each way, with you pay £37..40- so a £17.00 saving each way.

I'm a big believer in saving money, but I don't think saving £34 (for the round trip) justifies purchase of a £30 Family and Friends rail card. They'd need to weigh the minor net savings against the hassle involved in acquiring the card, assuming the only regular rail trips they'll take will be into town from Gatwick and back out there.

Frank, we are staying at a hotel close to St. Pancras station, and we will be backpacking.

Start with getting to your hotel near St. Pancras. The Thameslink train is direct from Gatwick to St. Pancras, and tickets for the 7 of you (3 adults and 4 children 15 and under) come to £35 total. That is the online price for Anytime tickets bought today for travel tomorrow.

https://www.thameslinkrailway.com/journey/london-st-pancras-to-gatwick-airport

There might be cheaper options, but they will be more complicated, and in this case simplicity is the key to travel happiness.

You can figure out your best way of touring for the rest of your day later. With contactless payment you can easily ride the buses which are fun and cheap, but that gets complicated with a group, and I don’t know how that works, since you cannot put multiple people on one card. Since you are returning to London for a brief stay later, Oyster cards might make sense, but again, that gets complicated.

Here are the bus fares:

https://tfl.gov.uk/fares/find-fares/bus-and-tram-fares#on-this-page-1

The 2 youngest. Children are free on buses and do not need a Zip Oystercard.

The next 2 (11-15) are free on the buses but do need Zip Oyster cards.

The 17-year old is an adult, so you have 3 adults. If you happen to have 3 contactless payment cards (Visa, AmEx, etc.) or ApplePay on phones, these 3 do not need to buy Oyster cards.

And here is a picture map of central London bus routes:

https://content.tfl.gov.uk/bus-route-maps/key-bus-routes-in-central-london.pdf

You will see 3 buses that stop at St.Pancras/Kings Cross, and 2 more at nearby Russel Square.

You can "duplicate" a credit card by using it across multiple phones set up for contactless payment, yes? Each phone becomes a unique payment method. Do the older kids have phones? For the bus, you might be able to get by with two standard Oyster cards. Or use the credit cards for the two kids that don't ride free. You're only in London for two days, after all.

Thank you for all the ideas and suggestions, it will definitely help us with planning! And yes, the teens have phones so that should be helpful too. If I'm understanding right, we could each use the same credit card on our individual phones as a tap to pay and it would work as different payment methods?

it would work as different payment methods?

yes.

You could also use linked watches.