It wasn't the witch
It was not the witch. I actually think the emaciated thing they saw in the forest was not Elly herself, but rather how she chose to appear to these kids in order to scare them and get them where she wanted. She could've chosen any other form to lure them (a beautiful young woman, an animal) but opted for the slenderman-like thing. Remember how in The Witch" the witch could take any form to lure her victima? It's what happened here, just think abiut it.
We've been waiting for you, Caroline...Waiting for you to believe.