Children travel for free on transport for london

>=11 year olds need to for example pay for the tube: https://tfl.gov.uk/fares/free-and-discounted-travel.

> Good luck finding out ahead of time whether your foreign card is going to be accepted by the system.

On the tills themselves there's all the signs of all the accepted cards and it was a fairly exhaustive list. It's also probably available online, so it should be pretty easy to check ahead of time.

"Many contactless cards issued outside the UK can be used to pay as you go for travel (overseas transaction fees may apply):

American Express (AMEX) MasterCard and Maestro (some cards issued in the USA, Canada and the Netherlands aren't accepted) Visa and V PAY (some cards issued outside the UK aren't accepted)"

So no you can't tell ahead of time.

You can still purchase a paper NFC ticket.

In Britain banks will give debit cards to children from age 11, and prepaid cards from age 6.

Although to get the child rate (half price) you need to show ID (if it's not obvious) and get the native Oyster card.