Being a devil's advocate for a moment, why do they respond in the first place about taking photos of kids ?

It's not illegal it's not a problem and not the police's business, it's just fuelling suspicion.

IF every complaint like this got 2 questions from the call centre?

1/ is the kid dressed ?
2/ is the kid in public ?

if answer to both is yes, then why dont the police just say "nothing wrong here, thanks" and do not respond.

When they respond then the public automatically SEE a problem.