Your child's left kidney
is very small and most likely it will be non functioning. But it needs to be evaluated for the cause. Since there are no stones, mass other causes has to be ruled out. Your child needs urine examination to see pus cells in urine which indicates infection. Then possibility of reflux of urine in to the kidney should be suspected.
Most likely possibility in your child is a dysplastic kidney which means kidney is not developed properly. In that case nothing needs to be done unless that kidney is giving any trouble.