Rachel St. Gelais
According to edwardemmett09 from the lion_lamb LJ community,
I'm guessing they are using her size (Rachel's) and her body for a younger Renesmee, then superimposing/morphing Mackenzie Foy's face on her body, like they did with Armie Hammer so he was able to play both Winklevoss twins (they used the body of a Ralph Lauren model for the second twin, then Armie acted both parts and...you have twins) in The Social Network. I guess this where the experts like John Bruno come in.
via Spunk Ransom