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?
It seems from the Shulchan Aruch Yoreh Deah 81:7 that one has to stop nursing by age 4 or 5 if child is weak.
The Shulchan Aruch in 167:19, see also Mishna Brurah 269:1 who holds the age of chinuch is 6 or 7.
so it seems that a bracha should not be recited at all.
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.