A new red ring that is getting larger on your child’s arm deserves medical attention, even if it doesn’t itch or hurt. There are several possible causes—including irritation, a fungal rash, an insect bite, or a skin infection—and the appearance alone isn’t enough to diagnose it.
If the ring is expanding, I’d contact her pediatrician today or use urgent care, particularly if you don’t know what caused it. The American Academy of Pediatrics recommends prompt evaluation when a child’s redness is spreading or worsening. (HealthyChildren.org)
Go to emergency care now if she has trouble breathing, becomes unusually sleepy or difficult to wake, seems seriously ill, develops severe pain, or develops a rapidly spreading purple/red rash. (HealthyChildren.org)
If she recently walked in wooded or grassy areas or may have had a tick attached, tell the doctor—that detail is particularly important when an expanding circular rash appears.
Until she is evaluated, take a clear photo with something for scale (such as a ruler) and mark the outer edge of the redness with a pen and the time. This can help you tell whether it is continuing to expand.
If you upload a photo of the ring, I can describe its visible features and explain what possibilities are worth discussing with her doctor.