When in doubt, the conservative approach is to continue breastfeeding.
The importance of breastfeeding does not diminish because the twins
are 1 year old.  Indeed, in my opinion, the most important benefits of
breastfeeding occur at the age when breastmilk is not so important any
more.

However, here is a chance to do some research.  The reason so little
data is available is the methylphenidate is rarely used in adults.
Thus, no studies of excretion into milk.  Team up with your local
clinical pharmacology department and start some publishing.

Jack Newman, MD, FRCPC