Depends on the investigation (IMO). If they have a suspect in mind they might want to try to keep that preliminary info quiet until an arrest was made so that the suspect doesn't have any reason to run or destroy evidence. And also so they don't get any false confessions. That's why people always wonder when an autopsy takes a long time if the delay is because they are trying to arrest a suspect first. If they have cleared the parents, they may tell them right away but I think most investigators error on the side of caution and not tell them anything until they are sure (and they may have to wait on the autopsy). I mean, if he was shot with a gun or something really obvious they might tell the parents that there were signs of foul play without revealing any details. But if he has injuries that are not clear like bruises and scratches it may be impossible to tell them anything without waiting for the final autopsy results.
Most the time I see them not telling anyone anything in order to protect the investigation. But this also protects innocent family too IMO. Can you imagine the extra grief and trauma it would cause to tell the parents there were signs of foul play only to have the medical examiner's final notes indicate trauma to the body was caused after death by an animal or the elements? On the other hand, if the parents have been cleared, then LE may want to tell them before the autopsy is released so that they don't hear the info on the news.
It's always a balancing act, in my opinion, because the autopsy is public record (unless sealed by a judge due to an ongoing criminal investigation) and the media have most likely already filed an FOIA request to get it as soon as it is ready. I feel like they also try to delay releasing autopsy results until after a funeral out of respect for the family. Especially with a child. MOO.