Visible teeth are in no way required in a portrait in my opinion. My favorite portraits don’t even have smiles. But when the smiles are there, make sure they look as good as they can. Studio light reveals everything and the bright white background in a highkey portrait can make even the brightest smile look yellow. Whitening the teeth a little makes a big, big difference on the portrait as a whole. It’s most important on color portraits of course, but even in black and white portraits you want that smile to shine.
The youngest children, until the age of 8 or 9, usually don’t need any whitening of the teeth. Only minimal if any. I don’t know why but in my experience boys from around 10 years tend to have significantly more yellow teeth than girls. You can whiten teeth in many different ways depending on the tools you use. I usually do it directly in Adobe Camera Raw with the Adjustment Brush where I have a preset that increases the exposure a little and decreases the saturation. Then I just paint over the visible part of the teeth and maybe adjust the exposure and brightness if it becomes too prominent.
Many kids have been told to brush their teeth thoroughly right before going to the photographer. Thus it is not unusual that their gums are darker red at the edge and maybe even bleeding a little. Make sure that you fix that too in your postprocessing – especially on color portraits.
