What's wrong with the approach of total honesty, just laying all your feelings on the table and letting her decide if she can handle it? If you're not able to do that, you're not ready for kids OR poly, both of which will test your relationship and your combustion skills to the limit.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.