I've traveled to London quite a bit, and the previous poster has given some GREAT advice. What I'd advise is think about how much you think you would need for a trip to NYC with kids, in US dollars. After you get that number, then multiply it by the current exchange rate for Pound Sterling. For example, if you think most kids would need $200 in the US, the current exchange rate for the British Pound is 1.65, so the kids would need $330. In my experience traveling to London, it seemed to me that everything was priced very similiarly to what we would pay in Dollars, in Pounds. For example, a bottle of water in the US is $2.50, as the previous poster indicated in London it would be £2.50, which equates to $4.13 US. Try to come up with what you think a good number would be for traveling to any big US city, and then add about $50 to it for a cushion, and then multiply by the exchange rate. It really is the best way in my opinion.