I wanna get from Beijing airport (T3) to Qianmen street, on Monday in the morning (about 8 a.m.).

Well, the peak hours in the morning usually last from 7:00 to 9:00. In order to avoid traffic jam, you may take airport express to Dongzhimen first. Then transfer to subway line 2 to Qianmen Station. The whole trip takes about 45 minutes. And the fare is CNY 29 per person.
By the way, if your child is less than 3.9 feet (1.3 meters) tall, he/she can ride free. :)