It would cost about 130 Yuan by taxi from the airport to Zhongguancun. You can take any taxi. You can take subway Line 4 at Zhongguancun Station to Xizhimen and then line 2 (one stop to Jishuitan), walk to Deshengmen to take special bus 919 to the Great Wall. For you have kid with you, taxi is the better choice. It will cost within 150yuan from the city center tl the Great Wall by taxi. For your three days tour, youc an also visit Forbidden city, Temple of Heaven, Beijing Zoo for Panda. The Great Wall will take nearly one day. In last day, you may have some relax in Houhai, Nanluogu Xiang to see hutongs, etc.