Final answer:
Forchheimer sign in a child with rubella is marked by the presence of red spots on the soft palate.
Step-by-step explanation:
The nurse should expect to note red spots on the soft palate in a child with Forchheimer sign, which is a characteristic symptom of rubella (German measles). The rash of rubella typically begins on the face and spreads to the extremities, but it is less intense and shorter lived compared to measles. Additionally, rubella is not associated with joint pain, enlarged lymph nodes, or a blueberry muffin rash.