Hi dr my almost 2year old has a black line on her thumb. should i be worried?

On the nail ?: Nail discolorations can be long term and a sign of disease or dietary deficiency, suggest you take you child to a dermatologist and see if this is in need of further investigations

Depends on where: Where on the thumb do you mean? You can show her thumb to her doctor who will likely be able to advise you further as to what it is, what is the cause, and if not, consider a dermatology visit as suggested by Dr. Goldenberg. You may also be able to do this via telemedicine with her doctor or on the Healthtap Medical Group section on our website.