I don't think it's that they're no longer credible, it's that they're over-used - familiarity breeds contempt and all that. Which would explain why they're still terrifying to kids - youngsters haven't encountered them enough times yet to have had enough of them. To me it kind of feels like the writers are a bit 'What do we do here? Oh, I know, let's just wheel out the daleks again.'
They need to be taken out of the picture for a nice long time and brought back when least expected. The first encounter with a dalek in New Who (I think with Rose and Christopher Ecclestone's Dr) was scary because it was so shocking, it simply shouldn't have been possible because the daleks were meant to have been totally wiped out. It worked brilliantly. And I say that as someone who couldn't stand CE's Dr. Since then, though, it's just become expected that the daleks will turn up and it's all a bit dull.
Dalek was a magnificent episode. Easily one of the best ever done.