Measurement invariance is a prerequisite when comparing different groups of individuals or when studying a group of individuals across time. This assures that the same construct is assessed without measurement artifacts. This investigation applied a novel approach of simultaneous parameter linking to cross-sectional and longitudinal measures of the construct of positive family relationships. Previously, a scale to measure this construct in mothers was developed longitudinally using the nominal response model of item response theory. In this study, this methodology was conducted for the first time to develop such a scale for children. The data for both informants derived from the Fullerton Longitudinal Study and encompassed 9 annual assessments spanning 8-years (age 9-17 years). This permitted linking across informants studied concurrently and prospectively. This procedure minimized measurement error, furnished a common metric across informants and time and established measurement invariance. Resulting thetas revealed a significant degree of concordance between informants across assessment waves as well as stability of individual differences for both informants over time. This psychometric investigation is unique because it simultaneously established invariance of a construct across informants and time. Implications for future research are discussed.