of course. But the Crime Commission doesn't routinely involve themselves in ~normal~ murders, only those associated with organised crime*.
The only thing I can think of is if the police do suspect the foster carers, then the money they've raised through the Where's William campaigns might be seen as a 'proceeds of crime' situation, and the Crime Commission does have under its purview, the recovery of assets of crime. Running a campaign to raise money to help find a child you'd murdered would be fraud.
I'm writing hypothetically here and am not accusing anyone of anything, I'm just putting that forward as maybe where the police are going with all this.
*yes there are other cases eg cases where there has been a police inquiry