Read other letters about this article
...which I'm too lazy to track down again, but you can Google up easily enough. The core of their argument is that the pictures in question do not in fact meet the Pennsylvania definition of child pornography which requires either exposure of the genitals or the Potter Stewartesqe I-know-it-when-I-see-it "lewd and lascivious intent" or somesuch. And they're not requesting money damages from the prosecutor (except for fees and costs), but rather injunctive relief: a declaration that the pics are not pornographic, the attempt to force the kids into diversion is a violation of parental rights and the threat of charges retaliation for resistance. Thus I'm not sure how qualified immunity comes into play.
Oh, and TC-F, the claim of interference with parental rights is based on the Fourteenth, not Fourth amendment.