It’s not ideal, I agree, but I think the only calculations so far programmed in are for full adult fares. It is mentioned in a way on: https://tfl.gov.uk/fares/find-fares/tube-and-rail-fares/pay-as-you-go-caps
Weekly cap
A weekly cap limits how much you pay for all your journeys in a fixed Monday to Sunday period. It works using adult rate pay as you go on contactless (card or device) or Oyster.