I'm not sure how to critique this as I can see no way to improve it. This is one of those rare cases where the emotional aspect of the subject matter imbues the work with far more than a straight forward portrait would ever have. Your choice of a loose, impressionistic style is inspired as it cloaks the image in ambiguity, making the subject seem not entirely there, as if lost, even in the paint. The heavy, textured paint adds to this effect, suggesting the shattered lives that arise when a child goes missing.

There are very few series that seem to hit all the notes in perfect tune. Your series of missing children is a fantastic example of one that does.

Finally, I recently saw a comment on a DA forum asking if art was pointless. I wish I'd thought at the time to direct the poster to your lost children. It would have very compellingly answered his question.