The proportions of children are quite different to that of adults. When they are born, babies' heads are extremely large in proportion to the rest of their body. As a child grows up, the head becomes smaller in proportion to the rest of the body.
The head of very young children makes up about a quarter of their total height. By the time they're six or seven, the head is about a sixth of their height.
A child's head is wider than it is long, so it's rounder than that of an adult. The features (nose, eyes, mouth) of a baby take up a very small part of the head, with the eyebrows being at about midway between crown and chin. There is virtually no chin and the nose isn't very prominent.
Humans are born with eyeballs at almost adult size, so a baby's eyes appear very large compared to the other features. Very little of the white of the eyes is visible.
As a child grows, the facial features 'move up' the face as teeth develop and the jaw grows to accommodate these. By the time a child loses their baby fat in their pre- or early teens, their eyes will have moved midway up the face.
If a child is looking too old in a portrait, it may be because the proportions and location of the features are those of an adult.