My preference is always to go to the source with questions like this - not "Rick" or another travel guide or blogger. Things change!
https://tfl.gov.uk/fares/ways-to-pay/pay-as-you-go
TFL tap and go has been around for since 2012 and 2014 - bus and later Tube and TFL rail.
Many of us prefer using credit cards (or Apple or Google Pay) instead of Oysters. Others on this board prefer Oysters. You can use the search function and find some of the discussions. Basically, some prefer Oyster - if lost, less important than the credit card. I prefer credit card - no need to constantly monitor the Oyster for cash balance and re-load. I have used ApplePay on my phone before the chip cards were common in the US - I find it awkward, but others have no problem. I've never had an Apple Watch - some like that for ease. Edit - the above post snuck in before I posted - good summary of on-going CC vs Oyster - consider it personal preference - valid arguments on both sides.
Depending on age, your grandchild may be eligible for discounted fares: https://tfl.gov.uk/fares/free-and-discounted-travel
Added observation - the contactless are becoming more and widespread across the world. I personally love it - I have a drawer full of transit cards from various cities that I can now discard and forget about packing!!!