I was also thinking about Bankside. But what about Doubletree Docklands Riverside? Sure a 5 year old would love arriving and departing on the ferry. There are also plenty of chain restaurants in and around Canary Wharf (I recommend Pizza Pilgrims), Museum of London Docklands (pre book the Mudlarks soft play), an easy hop down to Greenwich (similar deal for the Ahoy children’s bit, book online in advance), and of course “driving” the DLR (there is an ok kids playground in Millwall Park between Mudchute and Island Gardens DLR) . There is also currently skating at Canary Wharf (until 24 Feb) and the winter lights (from 17-27 Jan).