Hi
I recently studied at Saint Louis University in Madrid (la universidad de San Luis), and there were always postings for babysitters who were wanted. Practically every student on campus speaks both English and Spanish, and many would be more than happy to babysit. There are several public bulletin boards where you can place ads, and if you talk to one of the directors they may be able to put you in contact with a student they know who is looking. The address is:
Saint Louis University, Madrid Campus
Avda. Del Valle 34
28003 Madrid, Spain
and you can call for directions here:
Main Office: (34) 91-554-5858
I really hope this helps!