Hello, we'll arrive at 6.00 am at Capital Airport, have 10 h layover, can we visit Tiananmen Sq.?

Yes, you can.
It may take about half an hour when you deplane and get out of the airport. The time to and back from Tian'anmen Square to the airport may need about 2 hours. You can visit there for about 1.5 - 2 hours. Then you can also visit Jingshan Park. Remember to reserve 2 hours at least for your transit to the next flight. If you have bought the interline tickets and leave China for the third country, you enjoy a 144 hours visa free policy.

Thank you so much, that is very helpful.