回复：持美国绿卡,长期在中国,如何保留身份?

Keep renewing re-entry permit before it expires everytime. The re-entry permit can only be renewed (re-applied) when the green crad holder is physically in US.
The first two re-entry permits are valid for two years. After that, it may be only valid for 1 year.

They should keep filing 1040 every year or you should file 1040 including them as dependents.