How to keep depth of field but blur people

I would like to take a portrait of my child against a backdrop (Big Ben for example). Keeping my child and Big Ben in focus, how do I blur the people walking past my view?

You could get your kid to stand very still for say 5-10 seconds (I know this is hard with kids, it took me nearly 40 minutes to get the shot I done of my niece right) and do a long exposure, big ben and your child would be in focus and the people all blurred.

You could also use a tripod and a heavy ND (gray) filter. Take the picture with your child at normal exposure time (like 1/125th) and repeat the shot when your child is not in the scene with the gray filter with a long exposure time. Combine the two shots later on in PhotoShop.

Another approach is to use a long exposure time, but with a flash at close range of the child, so that only your child is lit by the flash. In this picture his/her face will be sharp, perhaps with some vagueness due to his/her movements, but the people walking in the street will be blurred. In this approach the child will be lighter than the background, giving that familiar daylight flash effect. If you do not want that, place the child under a tree, so that the child is balanced with the background.