Some people have pigmentation over elbows, knees, thighs and buttocks which is completely normal. Lot of skin changes occur as children grow, so no treatment is required at present.
Next Steps
Knuckle pigmentation might be indicative of
vitamin B12 deficiency. Please visit a paediatrician for the same.
Health Tips
Avoid vigorous rubbing in those areas as it is not dirt but thickened skin and it will only worsen the pigmentation.