There are no special qualifications to become a child behavioral therapist beyond the usual certification and licensing needed to become an applied behavioral therapist.
That means you follow the standard process for becoming a BCBA and getting licensed to practice as an ABA in your state, if your state is one of those with ABA licensing requirements.
If you are in private practice, you’ll also need a business license to be a child therapist. If you decide to apply with a school district or larger behavioral therapy clinic, they may require additional experience working specifically with children.