How much if any of the properties of prune juice will pass through the breastmilk to a child? if i drink it in moderation will my daughter benefit?

Most likely none: If your daughter has issues with constipation, it will not benefit her if you drink it. You can try pumping and mixing in a small amount of prune juice and bottle feeding her that.

Insignificant: It will probably not have a significant effect to benefit your infant for her bowel movements. You might hear of anecdotal reports of laxative effects, however, no studies have concluded connections with a mother's intake of prune juice and subsequently breastfeeding and later facilitating looser stools in infants.