The Parents had the daughter in their care since she was a tiny infant. Reportedly, they were going to be long term carers, in essence, until she was 18. If so, I would see that as her family. She called them mom and dad and lived with them 24/7.
An AVO does not automatically equal GUILT. There still has to be a trial. It surprises me that so few are willing to wait for the evidence before asking for the FP's heads.
The law does not see them as her family, and the fact that they are not technically her family is an important distinction. Fostering is not the same as adopting.
First of all, it means that they were subject to ongoing monitoring and reporting, and while this may have decreased as the arrangement became more stable, it would not have completely stopped. This is difficult to a typical family arrangement where, unless there is a specific reason for intervention, nobody monitors anything.
Second, at the time of William's disappearance, we know from various reports that the FPs were considering adoption. This adoption clearly never subsequently eventuated. So either they decided not to pursue it or they were told they would not be likely to succeed despite it having been implied previously that they would. This in and of itself raises questions about the degree to which FACS and the particular case workers saw them as suitable permanent parents.
Third, it is reasonable under the current circumstances of the charges and of FM being a POI to question whether William's sister may have spoken up about something that happened that day and whether the assault may have been punishment for that. If this is the case, and I'm not saying it is, but I am saying it's a valid thing to examine, it would mean that William's sister was manipulated for seven years. Such manipulation could also extend to indoctrination about her views of them as parents. It wouldn't be the first time - you would, I presume have seen this type of thing happen in custody cases.
Fourth, in my view it is the narrative that the FPs qualified as parents of both her and William that has contributed to obfuscating this situation and a lack of proper examination of what might have happened. If you turn it around and look at them as carers, as they were, it is much easier to objectively examine them.
ETA: The AVO does not indicate guilt of the other charges, but it meets a higher threshold than mere allegations or charges.