Spencer Breslin...Curtis the Elf
Spencer Breslin just managed to pull of his role in this movie (Santa Clause 3) but I don't know if he'll fit into the same role in Santa Clause 4.
He's older, taller, voice has gotten deeper and he has definitely lost the "baby fat" cheeks which made him an adorable child star (the changes are VERY noticeable if you watch Santa 1, 2 and 3 back-to-back). I keep watching his performance in Santa 3 and thinking "okay, he's a bit too old for this role...but the age increase has somewhat been explained by promoting him to the Chief Elf" (although Elves aren't really supposed to age)!
Spencer's still a very good actor so I'm hoping he will make the transition to adult actor well and have much success after this movie.