If you are thinking of going to Beijing for a vacation and you have some kids that are still shorter than 120cm you should be aware that kids less than 120cm are able to travel on the bus and subway network for free. Also many of the attractions in Beijing allow kids free entry if they are below this height.
Just recently my family had a vacation in Beijing. Lucky for us my daughter was 119cm at the time. As I mentioned this entitled her to free travel on the Beijing subway and bus network. We were also able to visit the Temple of Heaven with her gaining free entry. There are also other places you can visit that give free entry to short kids.
When we did travel on buses or the subway they were mostly crowded. One nice thing about the locals was that more often than not when we found ourselves on not being able to get a seat someone would offer my daughter their seat.

Inside the kids play area of the Science Museum in Beijing
If you are going to be visiting the home of the 2008 Olympics with your children two places my daughter enjoyed going to were the China Science and Technology Museum and Fundazzle. A science museum may not sound like much fun to a kid but if you ask my daughter it is. The museum has a building that is dedicated to the children where they can play and explore science. It is more aimed at entertainment rather than education.

Fundazzle was another great place to visit for our daughter. She actually went there for a birthday party. Fundazzle is a converted swimming pool. It has a large ball pit for jumping around and also a large climbing activity structure. The play areas are split up into one for they kids from 1-3 and another for the older kids.

The Science and Technology Museum is located at Bei San Huan Zhong Lu. It is actually within walking distance of the main Olympic stadium. Fundazzle is on the eastern corner of Workers Stadium.

