Chinese Visa or Travel Documents? 12 or 18?

My wife and I are holding US passports. We are planning to visit family in China this summer with 3 children. The 2 older ones were born when we had Chinese passports and no green card. Should we apply for Chinese entry permits for them or should we apply for travel documents for them? They went once on Travel Documents before we had our green cards. I have heard of cases that one side of the parents has US passport and their children was born before they had green card, and entry permits was issued to their children in 2014. And their children had travel documents before.

Is 12 or 18 the age limit for travel documents? Many thanks for your insight.
Min from PA