Parents are looking for individuals who are bright, fun and energetic. Having experience working with children is an asset. This can include babysitting, helping children in school, working for day camps or being an instructor of some kind (swimming, skating, piano etc). The ability to multi-task is essential.
It is not necessary to have experience with disabled children, as being a BI is very different than most experiences you would have in disability activities anyway.
The work of a Behaviour Interventionist is not a job, but a passion.