Individual Differences in SLA (Dornyei, 2010) refer to Personality, Aptitude, Motivation, Learning Styles, Learning Strategies, Anxiety, Creativity, Willingness to communicate, self-steem, Learner beliefs.
Motivation can be Instrumental, which refers to the drive to master a language for external purposes, such as a better job, and Integrative, driven by an intrinsic interest in foreign languages / a desire to interact socially with members of a target language community. This is irrelevant for kids because children don´t have a utilitarian motive, so research on this issue is impractical.