Should a child old enough to make brachos who is not weaned be encouraged to make a bracha on breastmilk? If so which one? Are there any direct sources for this?
Yes, but some adaptations must be made for technical reasons.
One should not say a beracha before the food or drink is before him
One may not say a beracha when there is an exposed breast present.
Ergo, one may not say a beracha on breast milk unless perhaps he first observes the breast, then goes behind a mechitza to say the beracha.