The book Nutrition During Lactation (IOM) says:

"Breastfed infants require approximately 30 min. of exposure to
sunlight per week if wearing only a diaper, or 2 hours per week if
fully clothed without a hat, to maintain normal serum
25-hydroxyholecalciferol levels.  Darkly pigmented infants require a
greater exposure to sunlight.......In summary exclusivly
breastfeeding results in normal infant bone mineral content when
maternal vitamin D status is adequate and the infant is regularly
exposed to sunlight.  If the infnat or mother is not exposed
regularly to sunlight or if the mother's intake of vitamin D is low,
supplements for the infant may be indicated."  See the book for
references and additional text.