Greenwich is amazing for kids. You could go into the observatory too if he is into that.
There's a playpark with pedaloes on the very shallow safe boating lake (probably only open in summer) and a cafe kiosk on site.
Maritime museum is free with some lovely interactive stuff - kids can load and unload freight from a crane, try semaphore signalling to each other, bring a boat into harbour or fight enemy warships using simulations.
The Cutty Sark tea clipper is beautiful and is fun to visit.
Take a ride on the cable car. If they have good heads for height, climb over the roof of the 02 (quite expensive and may have a minimum age.)
Take a boat ride on the Uberboat down to Westminster/London Eye past the Tower of London.
Walk under the Thames via Brunel's foot tunnel and steampunkish wooden elevator. At the other end side, ride Island Gardens DR - if the kids can get into the front seats of the train it feels like they are driving the train.
There's also a city farm at Mudchute (next stop up or a shortish walk from Island Gardens) if they like animals.
Loads of cafes around Greenwich market if they are hungry.
IMO Greenwich is the best place in London for entertaining kids.