In an interview with the timesonline, Russell T. Davis talks about the dark over tones of Doctor Who and how it affects Kids, saying: Kids love dark. I think it’s only adults watching who worry about that. Harry Potter gets as dark as hell in some places. All the great fairytales and myths are full of loss and longing. You’re an idiot if you remember childhood as just laughing, running about and having a good time. But I think it’s a job of good children’s drama to include the full range of emotions, which also includes happiness.
Read all his remarks at the link above.