Offer women alternative spaces where they can take their babies to breastfeed them and make sure that there is full and clear signage so that they know where to go and what for. To make it more subtle, add the fact that the space would also be a baby changing room.
Approach the woman and ask that she cover her breast and the child out of respect of others present. You recognize her right to feed her child, but showing her breasts in public is not included.
