For myself, I just swipe my mobile and I am done. For my child tried:
- Registering in the TfL mobile app doesn't work for children
- An Oyster Card for children should exist but we were told to get a paper ticket instead - at multiple TfL counters.
- Tried to swipe twice, for my child and myself. Did not work - unsurprisingly.
- Trying to get a paper ticket for a child leads to the mentioned pop-up, which says: "It's cheaper to use contactless [..] at adult rate [..]". What is that supposed to mean when trying to buy a ticket for a child? Is the contactless adult rate still cheaper than a paper child ticket? If not, then why am I seeing this pop-up?
So for our next visit: What is the proper way to travel with a 12 year old child as a tourist?