A localized gingival recession between two teeth in an otherwise healthy 7 year old child, may be due to:
Any localized gingival recession between two teeth in an otherwise healthy child may give rise to suspicion about abnormal oral habit. 
Many children have habit of stripping the gingival tissue or keeping the nail between two teeth.
This activity will cause gingival recession with respect to that tooth and if it is maintained for long duration has an adverse effect on the tooth-supporting structures.
ANUG
Abnormal oral habit
Chronic gingivitis
Herpes simplex