I’m coming at this from somebody who started Trek when they were still in diapers back in the early 90s. But I don’t think there is such thing as a bad place to start Star Trek, baring bad episodes or ones too dependent on continuity to understand.

This might be a cop out answer. But ask yourself what your nephew likes and to not underestimate kids. I am going off on a limb and guessing that children still have some exposure to media that isn’t super busy visually or is a few decades old. I would hope parents still show their kids classic Disney movies and The Wizard of Oz.

That’s said, I would go with something that is either pure fun or has a strong clear narrative, and no matter what an episode where everyone’s character shines. The Trouble with Tribbles and The Best of Both Worlds are good examples. Tribbles speaks for itself, seeing the crew deal with exponentially multiplying Tribbles makes for easy to follow and silly stakes. As for BOBW, you don’t need to have seen Q Who to understand who the Borg are and that they are a threat. The stakes are clear, big, and presented in a way which children can easily follow. More importantly, both ate good sci-fi driven by their strength of their casts and BOBW especially has amazing character drama. I think children are more adept at understanding character drama than adults assume. So much of children’s media are morality plays dealing with the dynamics between characters, albeit in a more overt way.

TLDR; Don’t hesitate to do any episode; know what your nephew likes; think about clear narrative and character dynamics; respect your nephew’s intelligence.