Breaking Dawn's Renesmee will be played by 10-year-old actress Mackenzie Foy. She is Renesmee. However, she will also be played by three other young actresses -- Eliza Faria will play Renesmee at age 5, Rachel St. Gelais will play Renesmee at age 6, and Sierra Pitkin will play the oldest Renesmee -- but not the way you might think. After all, it wouldn't be much fun to watch a character whose face is completely different every time she ages in a flash.
So special effects artists that also worked on Avatar, behind the movie are going to superimpose Mackenzie Foy's face on the three other actresses' bodies in the scenes where she is at varying ages/sizes. The digital effects used will be similar to those used on Brad Pitt in the movie The Curious Case of Benjamin Button.
This also means that there are more baby size Renesmees that have been cast, that we haven't heard of yet, to close the gap from birth to age 5. Or are we going from the birth to age 5 in a flash in Breaking Dawn? That is a little food for thought.