I believe that the sinful nature of a child should be obvious to anybody who has raised or been around children. This does not mean that they are sinners. Likewise, if Jesus did inherit the sinful nature of man through Adam it does not mean that He was a sinner, only that it would have been possible, even normal for Him to feel the temptation to sin (which, of course, as he "grew in stature and in grace", He did not sin).