Last Christmas we got my 6 year old son the Lego City 7723 Police Seaplane. This took him about an hour to assemble as it has over 200 pieces and it's quite involved, for example there are some fiddly bricks that attach the floats to the seaplane. It comes with a police pilot mini figure and a moving hatchway and large propellers that spin. It's a big kit and has just a couple of minor downsides - the tail can fall off easily and the floats can pop off during kiddy play.
Note that some effort is required to put the stickers onto the Seaplane accurately - I suspect this is beyond the skills of a small child to do correctly without making a mess as the stickers are a little unforgiving - stick once - and get it right!