I actually think it would be pretty hard to notice in the movie itself... unless you watch the credits.
credits on imdb.com:
Claire Geare ... Phillipa Cobb - 3 years
Magnus Nolan ... James Cobb - 20 monthsTaylor Geare ... Phillipa (5 years)
Johnathan Geare ... James (3 years)
It sure seems like they're wearing the same clothes and taking the same actions. Almost to give you the sense that something is off and it is probably fake... but giving you older actors so that it's not completely obvious.