I have a late diagnosed teen AuDHDer, who started in DBT when it was first thought that BPD might be an eventual diagnosis. A couple of thoughts -- yes, the child has to be 'bought in', so do parents. It's more like a curriculum than therapy, although the best therapists still find a way to connect with clients in a more traditional, personal way. My DD was barely bought in.. and it has been slow going at times, but there has been real progress with emotional regulation. She has completed her traditional course of DBT (explained below) but is continuing with her DBT therapist, staying true to DBT skills, but entering more of the traditional therapist/client role.
By authentic/traditional DBT I mean.. one on one weekly sessions the entire time, weekly parent/child group sessions (for at least 6 months), then weekly adolescent group sessions (no parents) for another several months, and likely parent DBT sessions (parents alone with a therapist.) It's a lot. And expensive. But many people say life changing. We're still in the thick of it.