What do they like to do? Which part of London? My children really liked going on the cable car over the Thames then catching the DLR (driverless trains!!!), then you could go to Greenwich Park and mooch round Greenwich Market. Or, Kew Gardens is huge and has loads of stuff to do and see for children (if you travel by train, get a 2-4-1 deal here as it is quite pricy). In central London, there is the Princess Diana playground which does get rammed but is worth the wait or, also in Hyde Park, the Princess Diana fountain which my children love splashing round in, it keeps them weirdly enthralled for hours. Science and Natural History museums are also close by should you fancy those.
We did hms Belfast recently. Boys aged 8 and 6 loved it. Also keen on south bank. Did monument and v keen to do London eye. The British museum and Wallace collection (suits of armour) both great. Imperial war museum if the are into tanks planes etc.