Hi. Greetings. IF YOU FEEL THAT THE SKIN IS UNUSUALLY DARK OR SUDDENLY TURNING SO, THEN SEE A QUALIFIED DERMATOLOGIST NEAR YOU TO RULE OUT ANY HORMONAL OR METABOLIC ISSUES. IF YOU ARE WORRIED ABOUT FAIRNESS OF THE CHILD - I WOULD SAY Leave the complexion and please let the child grow the way it wants and let it appreciate the colour given to him by nature and God. Also it could be constitutional or genetic since the parents may be dark or there may be a possibility that someone higher in your ancestry is dark and the genes may have showed up now. Also genetic mutations can occur with any one.
It's wonderful if she's growing in a healthy manner. If you continue to think like this, you'll subconsciously inculcate in her a feeling that "fair colour is better" which may harm her psyche and may be harmful for the society in general the long run. I understand this is quite a debatable topic, but for now, the best advise is to allow her to grow in a healthy way. Also you could protect hER skin and prevent darkening due to sub damage by using a sunscreen when she grows and plays out doors. Hope this helps