Neither of them are Kamina so why is there a Kamina tag.

Becuase the right one IS Kamina.

How is this genderswap?
It just looks like shotas to me.

It does indeed say "Kamina" in the Pixiv tags, but this looks like shota Simon and genderswapped Simon.

Neither are Kamina. Neither are genderswapped. It's normal-verse Simon and school-verse Simon. (If the one on the right WAS gender-swapped, he'd be wearing a girl's school uniform.)

I can't figure why the Pixiv entry has a Kamina tag, though.

Anybody can edit the tags on pixiv.

Yeah, but the tags added by the actual artist are marked with an asterisk. And anyway, there isn't a Kamina tag on pixiv anymore; it's two genderswapped Simons, one for each of the books the artist put out for C77.