Thank you for your query
There are various possibilities for this description.
1. A sacral dimple: It may be normal in many individuals. However it makes one prone to pilonidal sinus
(sinus formed due to hair in growth in to the cleft in the bums. It is even more problematic if the the person grows to have a lot of hair on the hips. However there no need to pursue and treat this
2. A sacral pit. It is deeper than just a dimple and indicates some anomaly in the vertebral
column of the child. In that case you should start with a x ray
lumbo-sacral region (lower back x-ray) and then if there is any doubt MRI
may be done. Further treatment depends on the findings. If they are normal nothing is to be done
3. A pilonidal sinus is a collection of hair in this region. It extremely rare at the this age and usually affects young adults and so we can skip this.
However for a proper diagnosis and further evaluation you should visit a pediatric surgeon
Hope that helps
God bless you