If your mod comes with a data folder, extract/drag it into your "The Elder Scrolls V Skyrim" folder. A data folder should be visible.
If your mod comes with Mesh, Textures, Sounds, or is an ESP file and doesn't have a data folder already; simply open your data folder and drop/extract the files th...
The second sentence on the Killable Children mod description.
"Also lets you play as them if you create a new character as a child with the mod enabled, a funny, unintentional bug. :P"