It has been listed on IMDb that Eliza Faria will portray ‘little Renesmee’ in the Breaking Dawn movie.
With that said, Mackenziefoy.com give us some information on how they will perform the CGI technique:
“We now have more info about the CGI techniques that are going to be used in Breaking Dawn to rapidly age Mackenzie’s character Renesmee. The CGI team is going to use a mixture of ‘Motion Capture’ and ‘Live Action’ Photography by fitting Mackenzie with an individually made skull cap fitted with a tiny camera positioned in front of her face. Like Avatar, the information collected about her facial expressions and eyes is then transmitted to computers.
As reported back in October Mackenzie’s face will then be digitally transferred onto the face and body of a younger child to create the illusion of her character’s rapid ageing. Besides the performance capture data which will be transferred directly to the computers, numerous reference cameras will give the digital artists multiple angles of each performance. All live action and motion capture footage will then be composited on screen.”