No matter what you do you just aren't going to get much out of the first day. Even the later arrivals aren't that great. Last year at about 4 pm after we arrived in London around noon my 10 year old had an incident that has entered into family lore as the "Cheese Shop Melt Down." She started sobbing in the Neal's Yard shop and said "I'm so tired, I just want to go back to the hotel and go to beeeeeedddddd."
PS My then 7 and 10 year old loved the Westminster by Gaslight walk wt
www.walks.com