Is this red line appearing on his skin? What are the dimensions of the mark? Did he eat anything he is allergic to? Or did you change your soap/laundry detergent recently?

my daughter has a red line on her arm and it looks as if it is spreading to her hand. It appears to be a lot of red dots but nothing is raised. Should I be concerned?

I'd suggest that she see a dermatologist. It's possible that anything could be causing the mark, from an allergy to an infection to a parasite...and you want to get it diagnosed and treated ASAP.