Did the kid get left in the subway?

No, on the subway tracks robo-woman has a sympathetic moment and tosses the kid to the scientist mom. Hines then shoots her in the eye just before a subway train plows into her and Hines falls between the tracks to avoid the same fate. Yeah I know, there's a lot going on there, and guess what? Robo-woman takes a lickin' and keeps on tickin' as she attacks once more, science mom tells her son to run to the street before finally putting Robo-woman down for good. In the end science mom helps Hines walk out of the subway.

I seem to remember that it's not her being sympathetic as much as it's just her enacting one of the memories of throwing her kid up in the air when he was much smaller.