How do you find a trustworthy babysitter/nanny in mexico city?

Question

someone willing to work 3 - 4 hours a day.

Answer 1

It has to be someone you know or somebody knows, because there are not trustworthy unknown people in this planet.

Answer 2

Same way you find one anywhere else.  Ask friends, neighbors, relatives for recommendations.  Ask for references from any applicants.  Check to see if there are any employment agencies who provide nannies with training and references.  When you hire someone, have her spend several days in your home with you and the child so you can observe how she interacts with the child.