I was kinda curious what IND would say about working on a student visa:
from http://english.ind.nl/
Can I work in addition to my studies?
You are allowed to work, but no more than 10 hours per week. Instead of these 10 hours per week, you could also do seasonal work in June, July and August or have a work placement that does not take up more than 50% of the course of study. Your employer must hold a work permit on your behalf.
So that's settled then.
Where do you attend university? Some (most) student towns have a studentenuitzendbureau, especially for busy people with only a few hours to spare, like you. "Studentenuitzendbureau + city X" is a good term to google by the way...
Since you are allowed to work legally, why do "off the record" work? It tends to pay minimum wage or less, and you'll compromise your visa when caught. Personally I would't want to take that risk...