Relatively child friendly is still not child friendly.
The Empty Child, Blink, Silence in the Library, Dalek, any episode actually detailing the origin of the Cybermen - I would not let my children watch these episodes (and many others I can't be bothered to list) unless I was sure they could handle it. Most kids would get nightmares for weeks. And yet, because it's popular enough and the toys sell well enough I guess, Doctor Who gets a free pass.
I'm not complaining, I love Doctor Who. But just because of some more mature content buried in Firefly (companions, the reavers, etc. - it was given a TV-14 rating at the time from what I've found by the way) - we don't get to suggest a simple set of the Serenity? I consider that unfair.
But perhaps we should take this somewhere else, I think this is a tad off-topic.