I'm not expert on what young kids like, but I wouldn't think they'd find Tiananmen and the Forbidden City all that interesting. As the largest public square in the world, Tiananmen is impressive, as are all the government buildings that surround it and the monuments on it. Mao's tomb is interesting to visit on several levels, but I can't imagine taking a young child to see it. The Forbidden City is fascinating from the outside because . . . well, because it's the Forbidden City, a huge and impressive palace from which the Emperor of China and his court ruled. There are a number of small but very interesting museums within the Forbidden City but, again, I can't imagine they'd hold much interest for a 3-year old.

There is a large park in back of the Forbidden City, the name of which escapes me, that might be good for the kids. As I recall, there is a lake with pedal boots that you can rent, a small "mountain" with a Buddhist temple on the top that you can climb, and similar "active" things to do.

The Summer Palace is probably also a good bet -- it's a large park around a nice lake. We saw lots of kids there when we went and they all seemed to be enjoying themselves. I'm also not sure how appealing the Temple of Heaven would be for kids.

Again, from a "kids" perspective, I'd skip the Hutongs unless you take a bicycle rickshaw -- the outside of the hutongs themselves are very plain and not of much interest, except to notice the stone lions, dogs and drums by the doors which give an indication of the original owner's status, and the few hutongs open to the public are either in very dilapidated shape or set up as "do not touch" museums. The kids might enjoy the be ridden around in the bicycle rickshaw, though (and you'll get a terrific photo opportunity ).