Hi Black piggy;
I started building up my supply 1 month before I return to work. This is to give me ample time to familarise myself with the pumping procedure too.
As i pump 2 to 3 times a day at work, thus I did not offer my baby frozen milk. If you intend to freeze your milk, then it would be advisable for you to let your baby try defrost milk first as not all babies like the taste of defroze milk.
Guess it would be better if you bring the frozen milk to the chiller (for defrosting) the night before you bring them over to your nanny's place.
Hope the above comments helps.