The rise of digital technologies has increased interest in democratizing music creation, but current creativity support tools often prioritize literacy and education over meeting children's needs for casual creation. To address this, we conducted Participatory Design sessions with children aged 6-13 to explore their perceptions of casual music creation activities and identify elements of creative applications that support different expressions. Our study aimed to answer two key questions: (1) How do children perceive casual music creation activities and which elements of creative applications facilitate expression? and (2) What insights can inform the design of future casual music creation tools? Our findings indicate that children view casual music creation as involving diverse activities, with visuals aiding in understanding sounds, and engaging in various playful interactions leading to creative experiences. We present design implications based on our findings and introduce casual creation as "purposeful play". Furthermore, we discuss its implications for creative MIR.