London City Airport LCY

From the Cumberland hotel maybe a two-minute walk to Marble Arch station.

Then take an eastbound Central Line tube to Bank station, then the Docklands Light Railway to LCY.

Adult fare is £3.70 with a Contactless card. Let us know what time you're travelling for precise advice.

https://tfl.gov.uk/fares/find-fares/tube-and-rail-fares/single-fare-finder

Bear in mind that your 17 year old will be an adult for travel in London. The 14 year old can get half price if you get an Oyster card and ask for the Young Visitor Discount to be added to the card. The Oyster card will cost £7 so whether it's worth it will depend on how long you are in London.

First, stop using Rome to Rio. Very unreliable and quite often wrong.

If you use a smartphone, download the City Mapper app and use its directions.

It's actually quite easy to get from Marlble Arch to LCY as Ramblin On noted. It should take about 45 minutes but give yourself extra time as the Central Line has been having delays and is not running as frequent as before.

At LCY, there are no barriers to tap out of the tube/DLR. You will find them at the side near the entrance to the station. If you don't tap out, you will be charged the highest fare possible.

LCY is a small airport. Distances are short and there are no jetways. You will use stairs to board your plane.

Following up on this… we will stay in the Victoria Station area. How early does public transport run on a weekday to get to the airports, and how long would it take roughly to get to LCY early in the am vs. taking a cab at that time? For example: We’ve counted out a flight leaving at 6:30 am from LHR because I think this means we have no option other than a cab that early.

Public Transport starts day services at just after 5am. But there are large numbers of all night bus routes to all corners of the city. Tubes stop, buses dont.
From Victoria to LCY the fastest route, at 40 minutes, is District Line to Westminster, Jubilee Line to Canning town, then DLR. First train at 0518.

To LHR there is a night bus half hourly from Trafalgar Square. Or National Express buses run at 0215, 0240 and 0330. Those journeys take longer than their daytime equivalents because they serve lots of unusual coach stops in West London, sort of being an Elizabeth Line replacement service.

But members of this forum don't catch buses in the early hours of the morning, they stay at a Heathrow Hotel the previous night.

The other option is a car service that will pick you up at your hotel and drop you off at the airport. Less expensive than a taxi.

What time is your proposed flight out of LCY?