child actor; child actress
Refers to a child who performs in films, television, theater, etc.
She debuted as a child actress.
That child actor is very good at acting.
child's role; child's part
Refers to a role written for a child in a production.
This movie has two child roles.
She debuted as a child actor at the age of five.
It's unusual for an adult actor to play a child's role.
She went from being a child actor to becoming a skilled actress.
Photos from his days as a child actor are getting attention.
女優 means actress and is defined by gender, whereas 子役 is defined by the performer's age (a child, male or female).
子役 refers to a child performer while 脇役 refers to a supporting role of any age; both end in 役 but classify different things.
子役 names an actor's age category (a child performer) while 主役 names the importance of a role (the lead), so the two are easily conflated but not synonyms.
Compound of 子 (child) and 役 (role, part).