A child's portfolio has one job: show a casting director range, clearly, in about six images. Everything else is decoration.
What the six should be
A clean headshot with genuine expression. A second headshot with a different mood. A full-length frame showing posture and proportion. Two or three character or wardrobe looks. For dance or performance work, one movement frame.
Casting directors look at a portfolio for seconds. Thirty elaborate images do not read as more range; they read as harder to skim.
Why heavy retouching fails
The portfolio's purpose is to predict what walks into the room. Smooth a child's skin, brighten their eyes and slim their face, and the casting director is surprised in the wrong direction on the day. We remove transient things — a scratch, a bruise — and leave the child looking like themselves.
Getting real expression from a young child
By running the session as a game rather than a job, and shooting continuously instead of asking for a held smile. Sessions are kept short deliberately; a tired child photographs as a tired child.
A warning worth reading twice
If an agency has told you that you must use a specific photographer, or pay upfront for a portfolio as a condition of representation, look into it very carefully before paying anything.
Legitimate agencies make their money when a child is booked, not when a parent pays for photographs. Requiring a particular photographer is a common structure in arrangements that do not lead to work. We would rather lose the booking than be the studio at the end of one.
Updating
Children change fast. A portfolio older than about a year stops being useful, and for younger children the useful life is shorter still.