Part of the thigh shows to the side of the leg on the new years pants items:

Outfit Not Found

You can see the blue skin color showing

When the Right Leg toggle is off, less of it shows, but still a substantial amount. It doesn't really look any better. The top third of the showing portion isn't a part of the leg that can be turned off, so it shows no matter what.

Outfit Not Found

looks like the bit that's showing is because of what's usually cut out where the hand would normally "cover" in the default avatar pose

No, that would look different - more hand shaped. Here's an example:

Private Outfit

same outfit, pants with the cutout.

These chocolate bar pants definitely don't have the cutout; that's why I used a posed shirt with them.

minor issue with happy anniversary kumos wind-up toy pants has basically the same issue as these and has been waiting for a fix for quite some time; idk if it's a problem with an avatar base template the artist is using but hopefully they add a few pixels worth of hip to prevent this from happening to a third pant design.