My son has a 7yr. Old daughter by his former wife. At present, he has regular custody, and has an excellent relationship with his daughter. The ex-wife (who has since re-married and has another daughter by her new husband) is planning on moving to Central America, and wants to take my grand-daughter with her. My son is not planning on giving his permission for this to happen, and will fight his rights in court. As a grand mother, do I have any similar rights?

You can possibly go and stick up for your son (within reasonable court procedure) but
That is about it.