kidnapping?

Assuming the greencard holder obtains the proper travel visa for the child (the child is a U.S citizen so presumably they aren't a ctizen of the country where the mother holds citzenship) and that there isn't a seperation agreement that or other court order that prenets the child from leaving the country then there probably aren't any laws broken.

I don't think there is any way for a custodial parent to be charged with kidnapping of their own child.

That said, it'd pretty a pretty crappy thing to do. There also isn't anything that prevent her husband for filing and getting custody before she does. If that happens then she could be responsible for bring the child back to the U.S..

But the baby would be considered an illegal alien and subject to deportation.