A bit of fill would have turned this into a pro shot and made your subject "pop", as it is its a shot uncle johnny could have done. H
I'm going to disagree with you. Whether by intention or design, the OP has captured the mother and child in a quiet, comtemplative, and I think very suitable light. Fill light would have destroyed that. Had they been looking toward the camera in a more traditional portrait pose, than yes, but as-is, I think this works perfectly.
A bit of fill would have turned this into a pro shot and made your subject "pop", as it is its a shot uncle johnny could have done. H