I have the same holes on my lower back area. They are pilonidal holes and quite common. Most people just dont notice them. At development, our bodies skin forms around that area the last, and as a result, there can be some holes that develop. I personally have 3 and they are harmless.
Did your child ever have a spinal tap as they leave a hole it is done with a large needle ,my son has the same and he had a spinal tap done when he was a baby for meningitas , he did not have it we wre happy to be told.
i have a big hole inmy foot above the ankle like that it never hurt me so i just left it alone i think its just her body..i mean everyones unique but if u think its important go to her pcp to get it looked at