“The Faces Pain Scale for the self – assessment of the severity of pain experienced by children: development, initial validation and preliminary investigation for ratio scale properties”
Bieri D, Reeve RA, Champion GD, Addicoat L, Ziegler JB.
Pain. 1990 May;41(2):139-50.

Altogether 553 children (195 first graders, mean age 6.8 years, and 358 third graders, mean age 8.7 years) participated in the development of a self-report measure to assess the intensity of children's pain. The first step was the derivation, from children's drawings of facial expressions of pain, of 5 sets of 7 schematic faces depicting changes in severity of expressed pain from no pain to the most pain possible. With the set of faces that achieved the highest agreement in pain ordering, additional studies were conducted to determine whether the set had the properties of a scale. In one study, children rank-ordered the faces on 2 occasions, separated by 1 week. All 7 faces were correctly ranked by 64% (retest 1 week later, 61%) of grade 1 children and by 86% (retest 89%) of grade 3 children. In a second study, the faces were presented in all possible paired combinations. All 7 faces were correctly placed by 62% (retest 86%) of the younger and by 75% (retest 71%) of the older subjects. A third study asked children to place faces along a scale: a procedure allowing a check on the equality of intervals. The fourth study checked on whether pain was acting as an underlying construct for ordering the faces in memory. We asked whether children perceived the set as a scale by asking if memory for an ordered set of faces was more accurate than for a random set. The final study checked, with 6-year-old children, the test-retest reliability of ratings for recalled experiences of pain. Overall, the faces pain scale incorporates conventions used by children, has achieved strong agreement in the rank ordering of pain, has indications that the intervals are close to equal, and is treated by children as a scale. The test-retest data suggest that it may prove to be a reliable index over time of self-reported pain