Findout from INS
This is a tricky question..
I have heard ( I cannot validate it though) that child can remain outside the country for longer times also.. Example, to get education in their own country.. Also, one of the parents can also accompany them.. It is just like that you can go outside the country for 2 years in a stretch on pretext of re-location after GC approval. But only once in your lifetime..
But again, finout from your attorney or INS..