I don't think Historia had a child with Eren. The timing isn't lining up. That conversation they had was from before Eren left to infiltrate Marley, so at least 2 or 3 years have pased since then, and Historia is only 6+ months pregnant. It's most likely that dude that bullied her that got her preggers.
Btw, I hope Eren destroys everything. Isayama, pls onegaishimasuuuuu
That form gave me chills, it's so dope kyaaa