I am planning to use my contactless debit card on the underground or buses. I will be with my 11 year old. Presume she needs her own card, am I best to get an Oyster card and get the child discount applied, or get an 11-15 visitor zip card. We will only be in London for 3 days, and 1 day we will be travelling out to Watford junction but I can use my friends and family railcard for that journey.