I have read endless experiences like yours. As soon as there are children the inactive JW partner gets all concerned about the upbringing of the children and Armageddon and goes back to the religion.
Your girlfriend may think she mean what she says, but if she does not know why Watchtower is a cult, then she is always going to be vulnerable to slipping back in. Before you get married or have children I strongly advise insisting that she research the religion and its history, and explain why she knows how it is not a good environment for children. If she refuses, or cannot take criticism of the religion, then you will know she is still suffering the indoctrination and vulnerable to joining again. Triggers will be some tragedy in her life, or having children.