Via Nessie Mackenzie

It’s not yet clear how the diminutive actress, who’s appeared in one episode each of television shows FlashForward and ‘Til Death, will portray the character who ages a full 17 years in only seven. Sources have suggested that director Bill Condon will employ similar digital effects to those used by David Fincher in The Curious Case of Benjamin Button, where Foy’s face would be digitally transferred onto the face of a younger child. It’s also probable that the studio will hire a younger child in some capacity, too.

See the EW article here

What do you think about Mackenzi as Renesmee? She does look like she could be their child.