How is their behavior? You may need to teach them to look at your face first, sit in their chair, and find powerful motivators. Say, "DO this" for what they need to imitate. If you have done that, just have them imitate wide open mouth ("ah" and "OOOO". then you can move to syllables. I like Nancy Kaufman's apraxia treatment kit. Use token boards for visual reinforcement. Use picture trading for requests. Good luck!