I just found a really awesome place in Zurich for youths, which hasn't been mentioned in this forum before. It all started with me searching for letterpress and screenprinting courses/facilities, and what I found was even more awesome
I wish i knew of this 5 years ago, but better late than never.
The place is called the Jugendkulturhaus Dynamo (http://www.dynamo.ch/
), and it is just along the Limmat across from the Landesmuseum park. You probably have walked past the Chuechi am Wasser restaurant in the summer, and watched some kids blazing and whacking metal (that's the metal workshop available).
On top of it, they have a snowboard/ski workshop (where for 10CHF, under-28/20CHF over-28) you can carry out maintenance on your snowboard or skis - they have a sandpaper grinding machine for your edges and base, and all the wax and tools you need!
Also the screenprinting workshop with affordable course (160/190CHF) and use of facilities (5/8CHF per hour). A textile workshop with cloth, leather etc for sale and use of sewing machine (5/8CHF per hour). A buttons/badges workshop. Metal workshop also with courses available. A digital media studio. Dance courses, yoga courses, and lots more! http://www.dynamo.ch/werkstaetten
I've signed up for their screenprinting workshop for February (the teacher speaks English, so if anyone else is interested, sign up for the Feb 6/13/14 evening course and we can have it fully in English!)
Hope some of you will find this post helpful! I found a couple more links for workshops for children and will post it another time.