You should also contact a health care provider right away if: A child who is less than 12 weeks old has a fever of at least 100.4<SUP>o</SUP>F, or 38<SUP>o</SUP>C. A child older than 12 weeks has a fever of at least 104<SUP>o</SUP>F, or 40<SUP>o</SUP>C.