Someone asked about known serial killers in Australia who targeted children. I lost the original post somewhere. I can only think of one known to have solely targeted children. His name was Derek Percy and he was locked up at the time of Karlie and Khandalyce's deaths. He is also now deceased. Derek Percy had only one count of murder attributed to him in a court of law -- the murder of a 12 year old girl in 1969. But since that time police have attempted to link him to a number of other child murders/abductions between 1965 and 1969, while he was aged only 17-21. A coroner ruled him guilty of the murder of a 7 year old girl (Linda Stilwell) only a few days after his death in 2013. Another coronial inquest found that there was enough evidence to charge him for the murder of a 3 year old boy, although the DPP ended up deciding not to pursue charges (although police are pretty much satisfied it was him). Interestingly, he is also considered to have likely committed the Wanda Beach murders at the age of 17, and there is some quite compelling circumstantial evidence in relation to that. He is also a person of interest in the abduction and presumed murder of the Beaumont children. Less evidence for this one, but police did confirm that he was in Adelaide at the time.
Percy was actually found not guilty by reason of insanity, but in years to come psychiatrists said that this diagnosis was wrong and he was 100% sane and unable to be rehabilitated. They said his "inability" to recall the details of the murder was completely faked. Ironically, if he had been found properly guilty of his actions he would have been released in the 80s and probably murdered more children but by originally being assessed as "insane" they were able to keep him locked up until he died in 2013. Strange loophole, that.
Anyway, sorry for the off-topic post, just wanted to reply to the original question. Back on-topic, I'm also not convinced that a serial killer committed the murders just yet, I think it's just a more likely situation that they want to consider all options. Looking into whether any of Karlie's associates has committed similar crimes doesn't necessarily mean a search for a serial killer per se. It might just mean that they are looking for a pattern of extremely violent behaviour towards women and children. But again, who knows! I expect that if a serial killer is responsible this case is going to take a long time to solve, as it would likely mean that the murderer chose Karlie and Khandalyce at random and it will be difficult to find the link. On the other hand if it is a person that Karlie had some form of relationship with, other people will know and be able to testify to the behaviour that characterized that relationship, and would also be more likely to hold information about the murder. Serial killers generally work alone and don't confide in others although there are exceptions.