"But am I worried that some viewers might find Donna’s departure TOO sad? Not remotely. Not for a single second. I believe, hugely, massively, that TV isn’t there to make you smile. Drama certainly isn’t. That ending is devastating. I hope it’s never forgotten. I hope people cry for years. In 70 years’ time, kids watching it now will be in old folks’ homes, saying, ‘Oh, why couldn’t Donna Noble have remembered just one thing?!’ There’s this great misconception that the Slitheen are for kids, and episodes like ‘Human Nature’ and ‘The Family of Blood’ are for adults. In fact, adults can enjoy daft green monsters, and kids can appreciate emotional, grown-up drama. Pixar understands that perfectly. JK Rowling does. If kids are upset, then they’re feeling something, and kids feel things vividly. The death of a goldfish is like the end of the world. It’s keen, real and powerful for them. But that doesn’t make it something to be avoided. If they can reach that state through fiction, well, they’re actually experiencing something wonderful. And important."
Russell T Davies, The Writer’s Tale: The Final Chapter
(on a side note, my kids will most definitely be raised as Whovians and Potterites for this particular reason)