In The Force Awakens, we see that FN-2187's superior, Captain Phasma, is a woman.
Are there other female stormtroopers in any of the other Star Wars movies? What do we know about them? Do they have relationships, or children?
Yes, at least some.
According to Rebels show, there were.
Miller's "A New Dawn" had a female stormtrooper searching a bar (Chapter 22)
The female stormtrooper surveyed the cheering crowd.
Captain Phasma is definitely a stormtrooper (though a bloody officer), so yes.
She is described as being proficient with weapons and participates in combat directly, so she's a "real" stormtrooper, not an armchair general like General Hux.
Her Visual Dictionary page says there were female stormtroopers
She pays little heed to the outdated notions of inequality between genders, an idea common on undeveloped worlds. To her thinking, a female stormtrooper is nothing new at all. The anonymity provided by their armour concealed the fact that both men and women served the Galactic Empire as stormtroopers.
This seems to imply there weren't many female stormtroopers in First Order beside her, if any - which is confirmed by both the novelization (none are mentioned except her) as well as prequel book "Before the Awakening" (same situation).
In The Force Awakens, the stormtroper on Starkiller Base who reports to Ren about Rey's possible location is clearly female judging from the voice, though the WGA script doesn't say so:
Sir, sensors triggered in hangar 718. We're searching the area.
A bit less of an evidence, but we know Imperial service was open to women, including in high positions, even if it was rare.
Disney canon had Captain (when she first encountered Kannan in Miller's book) - and later Rear Admiral (by the time of "Aftermath" novel) - Rae Sloane
Disney canon ("Lost Stars" E7 prequel) had a female captain of the Star Destroyer which crashed on Jakku
EU had Admiral Daala, Tarkin's girlfriend and tactician extraordinair (at least in simulations); and going outside military per se, Intelligence Director Isard.
Yes, and shown explicitly in more recent canon sources. In an episode of Rebels (In The Name of the Rebellion), Ezra, Sabine, and Saw Gerrera board an Imperial freighter transporting a giant kyber crystal--unknown to them, being taken to the under-construction Death Star--and run into a force of Death Troopers that's commanded by a woman, DT-F16, who manages to be the last survivor of the unit.
This is revealed only by her voice (in this case, provided by the go-to voice artist for the female badass, Jennifer Hale).