Hi... Worried mother... This is not the way melanoma of the nail presents... First of all it is very uncommon in India. Looks like you are reading too much on Google. The black line is most likely to be dirt collected under the nail. Dip your toes and the baby's toes in water for 10 minutes and clean them thoroughly... It should be fine.
Next Steps
if you need any further advise or would like to discuss about melanoma then contact my clinic, Juvenis in Bombay.