Check if rash is blanching rash
Assess any other symptoms - headache, fever, aches and pains. Is rash raised or flat. (If no fever, then unlikely to be scarlet fever.)
Phone the parent, explain the situation, ask what they wanted me to do, but also ask if child had been in contact with any contagious illnesses.
Probably take a "wait and see" approach, if there were no other indicators of illness.
Possibly isolate the child in a separate tent overnight, near the adults, so that I could keep an eye on her. If necessary, zip her tent opening onto mine, so that she was close enough to not feel scared, but safeguarding was in place.
Check at her bedtime, and again at my bedtime, and if any concerns, check overnight.
Call parents first thing with an update and decide next steps.