I think there's a really simple answer to the "magic to look like a kid" scenario. It won't be an acceptable answer to some, for obvious reasons, but I think there's a very simple answer.

We don't need to create a logically consistent rule for this case because it will never happen. Nobody, at least no mainstream game developer will ever make a game where consenting adults use magic to look like kids when they have sex. I don't need a logical answer to this question because it will never need to be answered.

I'm fine with wildshape druid sex. I'm not fine with kidshape sex, even though the heuristic for what's okay and what isn't doesn't rule it out.

I'm not bothered by that inconsistency because it's not a question that will ever come up outside discussions like these.

Maybe another way to say it is there's a broad spectrum of okay to not okay, and things like wildshape druid sex are in the not-okay-adjacent area of transgressive stuff. The never okay area includes both actual bestiality (because it includes all non-con stuff), and it includes the magic to look like a kid example.

If anyone wants to make an argument in favor of the magic-to-look-like-a-kid, I'd be interested in hearing it, but I'm reasonably confident there's full agreement that that's not okay, even though by the heuristic that accepts the wildshape situation, it should be. It's fine that it's logically inconsistent.