Doctor Who baddies have sent generations of children cowering behind the sofa in excited terror – but critics have argued that the Doctor's most recent foe, sharply suited aliens The Silence, overstepped the line and that the programme has become too scary for children.

Vigorous online debate about the suitability of the bony-faced aliens – who can erase the memory of any encounter with them – has forced Steven Moffat, the show's writer and producer, to defend them. "I emphatically think that's not the case," Moffat said, explaining that he tests the stories on his two primary school age children. "Children like to be scared – like on a ghost train or a rollercoaster," he said. "They have always told each other ghost stories in the dark."

The level of scariness in Doctor Who is at the right level for younger viewers, according to Moffat: "This is the kind of fear that can be dealt with by a man with big hair and a bow tie." In any case, he said, The Silence aren't his scariest creation – that title should go to the Weeping Angels, the stone figures who sneak up to their victims when they turn their backs.

Concerns have also been raised over disappointing viewing figures for the new series. Initial figures suggested Doctor Who had shed a significant number of viewers, with the new series debuting with 6.5 million viewers, down from 7.9 million in 2010.

However, when time-shifted viewing (of the sort the Doctor himself would surely approve of) on personal video recorders such as Sky+ is taken into account, that number rises to 8.9 million so far. That compares with an equivalent 9.5 million for the first episode of the last series.

"It was a very hot Easter and viewing volumes were down across the whole of television," said Dan McGolpin, BBC1's head of scheduling. "But the people who missed it because of the good weather did watch it.

"Doctor Who viewers are very savvy and do watch, whether it's live, or on iPlayer, or on catch-up.""

Some have also questioned the complexity of the storylines – one of which involves the Doctor having died 200 years in the future. "My youngest son deduced everything. No, it's not complicated," said Moffat. "Of course you have to listen to it properly, and it's not something you should pop out and cook during – it's proper, demanding fiction. But go and read Harry Potter or Alex Rider; these big doorstop novels are too."

But for those still wondering whether The Silence could possibly be real and living among us – even if we can't remember them – this week's instalment is a rather more jolly affair. The Doctor and his companions end up on a 17th-century pirate ship captained by Hugh Bonneville.