A Minecraft mod. Downloads can be found on CurseForge and Modrinth.
Baby skeletons use their own texture files, which are dynamically copied at runtime from the vanilla originals, so they are still compatible with resource packs changing the normal adult textures.
This means those textures are not physical present in the mod jar, but can be overridden separately from the adult textures using a resource pack with files at the following locations:
tinyskeletons:textures/entity/skeleton/baby_bogged.png
tinyskeletons:textures/entity/skeleton/baby_skeleton.png
tinyskeletons:textures/entity/skeleton/baby_stray.png
tinyskeletons:textures/entity/skeleton/baby_wither_skeleton.png