If the H1B extension has been approved and all of the associated paperwork is consistent and in order, I don't expect that there would be an issue with H4 stamping. You state that she's already in H4 status, so as long as she is able to document her continued eligibility for that status I don't anticipate a problem at a port of entry - but she should be absolutely certain that everything is in order with his H1B and associated documentation. Her having children born in the U.S. does not make her H4 any more or less valid, or give her any right to enter the U.S. if the H4 is not valid.
(As you know, sometimes small details can have significant effects, and I can only work with what you have told me. If in doubt, her best bet is to consult an immigration lawyer.)

