This is the place to post questions and discussions on usage and style. The members of the Wordwizard Clubhouse will also often be able to help you to formulate that difficult letter.
the use of the past/preterit or the present perfect in the first verb surely changes everything. In the second example the event is linked to the present, so "she" could still have more children (or indeed "have still more children" - not the same meaning either). However one could argue that if this were indeed the speaker's intention, then he/she ought to have used "have become"...ACCESS_POST_ACTIONS
Signature: "Being an agnostic means all things are possible, even God, even the Holy Trinity. This world is so strange that anything may happen, or may not happen." Jorge Luis Borges
True for British English, but I think this kind of tense mixing is largely acceptable in most flavours of US English.