In order to promote children's collaborative musical creativity in new digital environments, we need a better understanding not only of the sound production capabilities provided by the new digital tools, but also of the interaction affordances involved. This study focuses on the interactional patterns emerging in children's musical creativity, comparing creative group processes on iPad tablet computers (with GarageBand software) to processes on traditional musical instruments. Both instrumentations were assigned to five groups of four 10-12-year-olds for creating sound landscapes for a "space" movie. The traditional instrument groups' processes were characterized by peer teaching as well as multimodal, improvisatory negotiations with rapid exchanges between the participants, both kinds of processes involving the intertwining of deictic expressions with hands-on musical demonstrations, and clear signs of group flow. By contrast, the tablet groups relied on solitary, parallel planning processes where possible coordinations between the participants took on a more abstract, conceptual form, at a remove from the actual musical ideas and their interplay. Also, there were far fewer signs of group flow than in the traditional instrument groups. In sum, the tablets did not seem to match traditional musical instruments in terms of their interactional and creative affordances. This may be because the traditional instruments offer richer textures of gestural and tactile qualities, visual cues, and spatial anchoring points for facilitating concrete musical interaction, and because the GarageBand software actually requires some reliance on abstract conceptual labels, channelling the participants' attention toward pre-planning rather than hands-on musical play. The results are problematized with a view to our decision to treat the tablet computer as akin to a musical instrument rather than as an action environment of its own.