The quality of family relations, observed during mother-father-infant triadic interactions, has been shown to be an important contributor to child social and affective development, beyond the quality of dyadic mother-child, father-child, and marital relationships. Triadic interactions have been well described in families with 3 month olds and older children using the Lausanne Trilogue Play (LTP). Little is known about the development of mother-father-baby interactions in the very 1st weeks postpartum, mostly because no specific observational setting or particular instrument had been designed to cover this age yet. To fill this gap, we adapted the LTP to create a new observational setting, namely the Diaper Change Play (DCP). Interactions are assessed using the Family Alliance Assessment Scales for DCP (FAAS-DCP). We present the validation of the DCP and its coding system, the FAAS-DCP. The three validation studies presented here (44 mother-father-child-triads) involve a sample of parents with 3-week-old infants recruited in two maternity wards (n = 32 and n = 12) in Switzerland. Infants from both sites were all healthy according to their APGAR scores, weight at birth, and scores on the NICU Network Neurobehavioral Scale (NNNS), which was additionally conducted on the twelve infants recruited in one of the maternity ward. Results showed that the "FAAS -DCP" coding system has good psychometric properties, with a good internal consistency and a satisfying reliability among the three independent raters. Finally, the "FAAS-DCP" scores on the interactive dimensions are comparable to the similar dimensions in the FAAS-LTP. The results showed that there is no statistically significant difference on scores between the "FAAS-DCP" and the "FAAS," which is consistent with previous studies underlying stability in triadic interaction patterns from pregnancy to 18 months. These first results indicated that the DCP is a promising observational setting, able to assess the development of the early family triadic functioning. The DCP and the FAAS-DCP offer to both clinicians and researchers a way to improve the understanding of the establishment of early family functioning as well as to study the young infant's triangular capacity. Perspectives for future research will be discussed.