Why was Liv spared from her parents' gruesome fate? Easy, the bots never figured out where she was.
Besides the villains' Villain Ball moments and the obvious Plot Armor of the protagonist, a plausible reason why the previously unstoppable animatronics were suddenly so easily slaughtered by the Janitor was due to complacency. The animatronics had for many years stuck to their end of the deal of hunting people only in the abandoned entertainment center. The vast majority of their victims who were trapped in the hunting zones likely had a moment of panic, couldn't think tactically nor defend themselves properly. This made easy killing for the animatronics who got used to chasing down panicked people, so much so that they couldn't adapt to the Janitor, their newest prospective victim who doesn't fear them and is strong enough to fight back.
Ozzie Ostrich was clearly clueless about self-defense after years of doing the attacking. After giving the Janitor a cut on his cheek, Ozzie had an Oh, Crap! moment when the enraged Janitor attacked him in return.
Cammy Chameleon was also bemused at the Janitor deciding to go for a break just when they are about to fight, so much so she did not attack immediately.
The beginning of the movie actually could support it: the animatronics were said to be very proactive twenty years before, stopping the demolition the morning before it's supposed to happen and happily breaking out of Willy's Wonderland to hunt for their victims, but Liv and her friends have no problem throwing gasoline on the building. They just got used to Lund and the town handling everything for them, and only enjoy the hunt.
Let's not forget who the spirits were: serial killers who preyed on children and unsuspecting families, and they committed mass suicide when they were about to get caught. As animatronics, they had the element of surprise by killing unsuspecting and defenseless victims and then scaring the town into subservience. They never faced someone who was neither afraid of them nor could fight back.
There are some hints that the Janitor may have been supernatural himself. He is completely unsurprised by the haunted animatronics, and his strange silence, the fact that he sticks exactly to the letter of his agreement, exact adherence to his breaks, and his remarkable cleaning ability are all indications that something is not exactly normal about him.
Siren Sara is actually a fairy rather than a siren. However, it could be a case of Meaningful Name in a more subtle way; in mythology, sirens would lure sailors to their nests and drown them. In that sense, Sara is absolutely a siren; she would use her fake charms and cheerful attitude to calm unsuspecting victims before killing them.
Even regarding her physical being, she actually more closely resembles the original idea of a siren, which were winged people in Greek Mythology. It wasn't until the Middle Ages the concept of sirens would morph into evil mermaids.
Liv had to witness her parents' gruesome murder and then their bodies being devoured by the bots and had to stay inside of her hiding spot until morning. That's not only horrifying but tragic.
Not just that; it was the Sheriff who raised Liv afterward, for what looks to be at least ten years. Liv tells the Janitor that growing up with her "was like being a ghost." How many of those years were after Liv figured out that the Sheriff helped set up her family to be slaughtered... but Liv had no way to leave?
