Day out in London, what to to with 6 & 10 year old

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My husband is taking our two girls to London for a day out with his sister between Xmas and New Year, any suggestions for where to go and what to do.?
Thanks

It depends what you've already done. People will say the museums, but they get packed, and i like being outdoors.
My cd liked this trip. Thames clipper at embankment to 02. ( it goes quite fast down river) Emirates cable car across Thames. DLR to Westfield and lunch. Then over to Olympic park ( great play area too)

*dc