A baby on Space Mountain really is a terrifying thought.
I'm wondering if OP actually saw the baby moving around? I ask, because (although I know it's summer and most kids are out of school - some attend summer school or year round schools...) my daughter was required to "take care of" an electronic baby for a class in high school, year before last. And while we only had to deal with it for a couple of days, at other high schools in the area they were making kids do it for a full week. I wonder if it's at all possible that the "baby" OP saw in a sling was actually an electronic doll, and that the girl carrying it just couldn't get out of taking the thing to
Disneyland for the day (remember, a lot of people are locals and just there for the day)???
It seems like a long shot, I know....but it still seems more plausible than a real live baby being "missed" and allowed on Space Mountain. The electronic baby we had to tote around with us looked very real, and the school sent my daughter home with items like a baby sling and infant car seat, etc.
On the other hand, during our trip last month we saw an hispanic family sneak a tiny toddler into the Splash Mountain line. The CM told them they couldn't bring him on, they acted as though they knew no english (maybe they really didn't, i don't know), and then when they acknowledged that they knew this tiny kid could not go on the ride (he was about a foot too short), they told him to go with an older woman in their group to wait outside the ride. The kid threw a fit, and when the CM wasn't looking, one of the adults in their group waved for him to run over and he was so tiny that the CM never saw him run behind and dart into the line (we did point it out to the CM, but she did nothing). I'm sure they caught him before he actually boarded the ride, but it really struck us that some people will endanger their kids just to put them on those rides.