What could be the concern for the new onset of dark veins in my 5 year old forehead?

Skin condition: It could be that your child's skin has thinned a bit ans the veins are more prominent. As the skull grows the skin can stretch. I don't think it is a cause for concern. See your pediatrician if it worsens.

Not sure: Without seeing your child, hard to say. If no other issues than the appearance of the veins, would not be too worried, but would make an appointment to see doctor for evaluation. Hope this helps. Best of luck. Dr R