Mild nail splitting/cracking can commonly occur due to dryness, repeated friction, nail biting/picking, or minor nutritional deficiencies. From the image, it does not look like a serious fungal infection or emergency concern.
Next Steps
Ensure adequate diet with protein, fruits, vegetables, milk/curd, eggs/pulses. Keep nails trimmed and apply moisturizer/coconut oil/Vaseline over nails at bedtime.
Health Tips
If child is otherwise active and growing, isolated mild nail cracking usually does not indicate major
vitamin deficiency. However, because birth weight was low and current weight is on lower side, a pediatric review for growth assessment and possible anemia/nutritional deficiency evaluation may be useful.