This is the accurate translation, but for your novel, "assistant therapist" might be sufficient?

En aktivitør arbeider med å aktivisere eldre på et dagsenter for eldre eller et alders- og sykehjem. Det kan også være aktivitører ansatt i omsorgen for psykisk utviklingshemmede eller funksjonshemmede, for eksempel på vernede bedrifter.
http://no.wikipedia.org/wiki/Aktivitør

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Note added at 5 hrs (2009-05-17 10:03:03 GMT)
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Forskjell på "Occupational Therapist" og "Occupational Therapy Assistant" (takk til Vedis!)

Registered Occupational Therapist (OTR)
The OTR is a graduate of an accredited professional masters or doctoral program and has passed a national certification examination. The OTR must also meet state licensure requirements. In addition to academic courses, the OTR student must complete six months of clinical internship. The OTR is responsible for evaluating patients, setting goals, providing treatment, and evaluating progress. They are responsible for supervision of the Certified Occupational Therapy Assistants. OTRs serve as practitioners, instructors, researchers, and administrators in education and clinical settings. Starting salaries of $38,000 - $55,000.

Certified Occupational Therapy Assistant (COTA)
The COTA is a graduate of an accredited technical program and has passed a national certification examination. The COTA must also meet state licensure requirements. The COTA program includes an
introduction to biological and behavioral sciences with an emphasis on use of media/activity and its application. Four months of internship is also required. The COTA works under the supervision of an OTR
and assists the OTR in evaluating and carrying out the treatment program. Starting salaries of $23,000 -$31,000
http://www.sdaho.org/Education/Career Brochures/OT.brochure....