IMO, the theory that makes the most sense here is that police has been suspecting foster family for months but have been missing solid evidence. Best option there for them was to hide those supicions, and watch them closely until they incriminated themselves in a conversation. What I think has happened is that surveillance has picked up an assault on the foster child. My guess is that police would have preferred to not intervene because they didn't have the info they still need for the WT case, but ethically they had to protect the foster child and remove them, which blew their surveillance operation.
Look at the wording in the article below.
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William Tyrrell: police strike force investigating disappearance charge man and woman with assault of a child)
That's totally in line with the assault being recorded or witnessed by surveillance.
IMO what's caused the recent flurry of activity is the fallout of the surveillance operation being blown. Removing the child from the family means revealing to the foster family their suspicions and the fact they've been watched, something the police had gone to great pains to hide until this point. So now police have had no choice but to move very quickly and prematurely even though they don't have enough information yet on where WT's body is or what happened. That theory would also make sense with the amount of drama around the digging etc... If you have been forced to reveal your suspicions before you were ready, you would want to make it look like you have enough incriminating evidence already to force the suspect to panic and make a mistake. Let's hope it works and find what's happened to WT, but I don't think things right now are unfolding the way police would ideally have wanted them to unfold.