This post contains spoilers for the movie Toy Story 3. It also contains spoilers for Toy Story and Toy Story 2, but if you haven't seen them by now, I assume you don't care.
The Toy Story movies are just cute movies about the secret life of toys, right? But consider, for a moment, the ramifications of toys as living beings. As viewers have seen, Woody, Buzz, and the gang are often subject to the same emotions of jealousy and fear that we all feel. In fact, it was Woody's jealousy of newcomer Buzz that spurred the first movie's adventure.
Other prime examples are the villains of Toy Story 2 & 3: Stinky Pete and Lotso. Pete became bitter from having sat on a shelf in his package, never taken home and experiencing a child's love. Lotso, on the other hand, had a special bond with a child named Daisy and courageously journeyed back to her side after being left behind, only to discover he had been replaced by a new Lotso. At that point he snapped, and eventually became the embittered overseer of toys at Sunnyside Daycare.
Perhaps the one thing that saves people from the vengence of such toys is the rule, never explicitly stated in words in the films, that humans must never know that toys are "alive". Where this rule comes from is a mystery, but even the freshly unpackaged Buzz Lightyear, who believes himself to be a real Space Ranger, follows it by going lifeless as soon as Andy shows up. It's almost like it's an innate part of being a toy. Except . . . .
As even the first movie showed, there's nothing magical about the transformation from animate to inanimate. It doesn't happen simply because people are present, it happens because the toys choose to do it when people are present. There's always the danger of being accidentally observed, as was teased in Toy Story 3 by the scene with the janitor in the bathroom.
Furthermore, the toys can choose not to follow this rule, as was the case at the end of Toy Story. All of the toys let Sid see them move on their own, with Woody lecturing him the whole time. And frankly, perhaps the toys didn't think the whole plan out. After scaring Sid silly, what do they think he's going to do? Yes, he'll probably stop torturing and destroying them, but he's also likely to throw all the toys in his room away as soon as possible to get them away from him. So in the end, all of Sid's mixed-up toys wind up in the incinerator at the Tri-County Landfill. But I digress.
So what happens when one day an embittered toy realizes there's nothing actually stopping it from taking it's revenge on the humans that wronged them? Imagine Lotso taking his revenge on Daisy by sneaking back in and suffocating her with his own plush body. Imagine the Child's Play movies without the complication of needing a voodoo ritual to inhabit a doll with a human soul. It's enough to make a kid never want to play with an anthropomorphic toy again.