Project-based assessment has been used to evaluate coding projects created by students for a long time. Nevertheless, there is a lack of rigorously tested project-based coding rubrics that are developmentally appropriate for early childhood. This study presented the development and testing of a coding rubric to evaluate children's creations with the popular ScratchJr app for early childhood, as well as results from field testing of the rubric. This paper first presents the ScratchJr Project Rubric development phases, and then a field test on 228 ScratchJr projects from 1st and 2nd grade students (n = 87, aged 6-7 years old) across three time points. The results showed that the rubric demonstrates validity and reliability, and can measure changes in the project quality across time points. While the rubric was designed for researchers and teachers to evaluate ScratchJr projects, the design and conceptual framework is applicable to other programming languages for children that invite creative coding.