Baby Player is an addon which makes it possibility to become a baby (also known as a small person). The best thing is that it works for any custom skin. This is purely a visual mod as it doesnt actually change any characteristics of the player (e.g. health or strength).
This is not an official Mojang product, and is not affiliated with Mojang AB in any way. The Minecraft Name, the Minecraft Brand and the Minecraft Assets are all property of Mojang AB or they respectful owner. This app follows the appropriate use guidelines at https://account.mojang.com/documents/brand_guidelines