While the use of musical parameters for emotional expression has been extensively studied, little is known about which specific musical parameters children at different ages are able to use for expressing specific emotions. We used a novel interface called Music Box that allows modification of musical parameters while music is being played in real time. Children (N = 37, 18 girls) at the age of 3 and 5 were asked to modify three parameters -- tempo, loudness, and pitch -- in expressing three emotions -- happiness, sadness, and anger. We hypothesised that both 5-year-olds and 3-year-olds could use each of the parameters in differentiating between the emotions. Results showed that 3-year-olds were equally competent than 5-year-olds in using the parameters. Differences based on gender and music background were additionally measured. The study provides knowledge on normative development of children's ability for musical expression of emotion, which can be used to inform music education and music therapy practices.