You want to search for Reittherapie...
One place to look, where you might find more information as to what is required:
http://www.ipth.ch/reittherapie.html
Google brings up several places offering riding-as-therapy in Switzerland.
Some years ago I looked for similar volunteer opportunities, as I used to volunteer with RDA in Hong Kong, working with physically and developmentally disabled children.
Unfortunately, I found that the organizations/riding schools I contacted here required specific qualifications, which I lack - I'm just a person who likes horses and wanted to help kids.

But if you were doing this as part of a school experience there might be more flexibility involved - can't hurt to ask.
Good luck with your search - it's a very fulfilling experience.
ETA:
Found this that might be of interest, although I don't know if it is current. But even if not, this might be a place to look for other opportunities:
"Für das Kinderspital Zürich suchen wir ab sofort in das Freiwilligenteam des Rehabilitationszentrums in Affoltern am Albis eine weitere Freiwillige Begleitperson / Pferdeführerin für die Hippotherapie..."
http://www.freiwilligenjob.ch/dnn/Fr...FachstelleKTZH