I'm taking the kids age 6 & 8 to London for a weekend. I'm a London novice, been a couple of times only. I'm worried about silly things like how to get on the right tube line, how to get us all through the gates, how do you know where places are in relation to each other up on the surface... I heard you don't need to get tickets or an oyster card for the underground these days, you just pay by phone and it's capped? Would dearly welcome any tips!