Get a visitors travel card, look on the tfl website.
You've not left yourself enough time to get a travel card for the kids so you'll have to get a paper travel card for the kids, it does allow unlimited travel for the duration you get it for.
Children aged under 11 can travel free on buses and trams at any time. They can also travel free on Tube, DLR and London Overground services at any time as long as they are accompanied by an adult using a valid Visitor Oyster card or ticket. Children aged under 5 travel free on the Emirates Air Line when accompanied by a fare paying adult. Children aged 5 to 15 pay child rate fares on the Emirates Air Line.
A maximum of four children under 11 can travel free with an adult who has a valid Visitor Oyster card or ticket on Tube, DLR and London Overground services.
If you’re using a Visitor Oyster card, you must touch in and touch out as normal but ask a member of staff to let your group through the side gate.
5-10 year olds pay fares on most National Rail services. They can get a 5-10 Oyster photocard to pay as they go at child rate which is cheaper than paying cash
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Pay child fares on bus, Tube, tram, DLR, London Overeground, National Rail and the Emirates Air Line. Children aged 11 to 15 years can get an 11-15 Oyster photocard to travel free on buses and trams and at the child pay as you go rate on Tube, DLR, London Overground and National Rail services. If you have not allowed a minimum of 4 weeks to apply, buy a child rate paper Travelcard instead