Christ, in true Byzantine form, is not depicted as a child, but has a mini-sized adult, which means to show he is not an ordinary child, but that he is fully God incarnate.
This is true, but to expand: Like the Sinai Pantokrator, the two natures of Christ are being depicted; here, a small child with certain "wizened" features shows that Christ is both a true infant and the Logos who sustains all things.
But we are not to understand this to mean that Christ was born with an adult mind hooked into "the Logossphere" for access to all knowledge in the ashakic records [replace with acceptable Greek analogue] or something. Christ was a real human infant with a real human infant mind, who developed and grew in wisdom and in stature. I believe this icon shows the authentic incarnate human reality of the Logos, alongside his eternal reality which he now expresses theanthropically.