Cheapest way to travel round London?

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Visiting London next week for 4 days but staying in Greenwich. Will want to travel around London to see the sights. Was going to tap in/out on tube but realised I can't do this for the children withoutpayingfull fare. What's the best/cheapest option please? Or am I best just sucking it up, getting them to tap and pay adult fare? Thanks.

@CruCru why don’t they beep then? It doesn’t cost them anything but would help towards data about passenger numbers etc.

Because when they get buses to and from school, the driver just waves them on to get the queue down quicker.

@CruCru the 9 year old technically doesn’t need to beep on buses but the 11 year old should so that’s a bit naughty of the driver.