Sorry that's just stupid.

IMDB lists Brown as born in November 1984, Halloweentown High released in October 2004. She was already around 19 while filming. The next film was set in college and released two years later. Hardly a difference makeup wouldn't overcome if at all necessary.

And the target demographic was clearly the young adults that grew up with the movies. Hence the logical continuation from high school to college. I doubt the casting cared for the actual age of the actress so far she could pass as student.

Way more likely is that the now grown up child stars are more self assured and learned from their older support cast that wages go up with subsequent entries.