I just introduced my kids to TOS (and Trek in general) tonight with Tribbles. Thought I'd go with light humour for the younger one as he's a bit prone to bad dreams if there are disturbing ideas or images too close to bedtime. My first episode was The Devil in the Dark. I was the same age as my younger child--7--but never had nightmares. I thought of going with Devil but the killing of the miner that I recall would likely have upset my son (in his dreams if not at that moment). I'll ease him into it. As for my daughter, she'd have had no issues at that age but I didn't have the TOS disc sets back then.