Defining categories of musical actions in improvisation with young children is challenging due to the spontaneous, creative and emergent nature of interactions. Following a literature review, two new constructs were proposed to circumscribe and classify different types of events in improvisation, Creative Musical Agency (CMA) and Socio-Musical Aptitude (S-MA). These were refined and tested through eight phases of mixed-methods research. Two cycles of improvisation workshops were video-recorded. Multimodal Video Analysis of musical, gaze and gestural Modes of Communication contextualized with field notes was used to refine constructs. Two raters independently observed and rated children's improvisations as showing CMA, S-MA or neither, giving reasons for difficulty or ambiguity in using constructs in separate interviews. Raters demonstrated fair agreement for CMA (?appa 0.21) and moderate agreement for S-MA (Kappa 0.5). Development of these constructs offers a valuable way of understanding the complexity of young children's musical actions and mental processes in improvisation.