I think "they" is preferred over "it", and not rude at all.
"How old are they?" "When did you have them?" "Are they a boy or a girl?" "They are adorable."
I would probably just drop the pronoun completely.
"How old?" "When did you give birth?" "Boy or girl?" "So adorable!"
Those sentences sound completely normal and grammatical to me.
Only if you are talking about more than one person. If you are talking about a single person those sentences sound bizarre.
Look up the "epicene they".