Well, there are women who lactate due to hormonal fluctuations even without a pregnancy occuring, but I think those are rare cases? Maybe (high) elves can lactate on demand with spirit magic? But is that even efficient? Mothers need to intake higher amounts of calories than usual when breastfeeding, so I don't quite get how that would help with saving on food costs.
If it becomes known that you can enable your child to use spirit magic and make it eligible for elven kinghood, then I think Laurier's and Sylvia's breasts would be in high demand.