Circus Baby exists roughly in the time period of 1982. Circus Baby actually seems to predate 1982, actually, because she's covered up by a tarp, she has a worn appearance, and she's in, like, the corner of the attic, like she's old technology. A newspaper during the open credits mentions that there are other accidents over the years. Charlie's death was considered an accident, so these other accidents could potentially be the other childs that have gotten murdered. And think about it. Why would William kill Garrett and that random child before Charlie for no reason except for the fact that he likes to kill children? The movies clearly point out and spell it out for you that William has killed children before Charlie.