It would probably be safe to say that the two children would get off the bus sometime between 2:45 and 3:30.
The sheriff deputy made the first wellness check sometime between 6:00pm and 8:00pm - lights on in the house, Tina's truck in the driveway, and no response.
Without a little more precise time for the first wellness check, we've still got a time frame of 3 to 5 hours for whatever happened in that house to have happened.
Since Sarah evidently didn't see Stephanie on Wednesday (last time she saw Stephanie was on Tuesday), we can assume that Stephanie wasn't at the house when the children arrived home from school. OR, that something had already happened to Stephanie and she was not seen by either of the two children when they arrived home.