There are two kinds of sight-reading at the piano: public and private. For public reading one must be prepared to sacrifice notes in order to keep a steady rhythm. For private reading one sacrifices the beat to the vivid color of the tones.
The question arises whether it is possible to train a child to be good at both kinds. That requires first of all knowing the difference between them.
The public kind, at which I excelled when young, favors quick movement; in my case it led readily to a short attention span. The private kind, of which I had no inkling as a young musician, yields in-depth understanding of the complexities of a work, therefore it lasts longer.
The difference may be compared to speed reading as opposed to reading for comprehension.