I concur, as an HH member, never put a small child into your calculation. Also, as you do not seem to have a compelling business reason I would suggest that you perhaps stay at any property but these, get your rooms for 50k or so a night and save a bundle. I would not pay twice as much for the Conrad St. James (totally dead there, where are you going to buy whatever thing you will need for the 18 year old? The convenience shop at the tube station across the street) nor the Hilton Mayfair (the least charming building in London), and would prefer a number of other neighborhoods.