Also, if it's a nice day, start at Trafalgar Square, walk down the Mall to Buckingham Palace, great photos!, through St James Park, feed the ducks, up to Parliament Square, Big Ben, Houses of Parliament - great photos again, up Whitehall, past Downing Street, GREAT photo opportunity for the London Eye there. That walk will probably take an hour at a stroll with kids.
That takes in lots of the big sights without it being too exhausting and gets you off the roads and into the park too so the kids can have a run. It's my standard walk round London when I have visitors from out of town.