Sometimes when a horse is pregnant and already malnourished, any nutrition poured into the mare goes directly to the foal, or so I've heard- and the mare really doesn't get much of it at all. As soon as that foal is born, up the mare's feed even more, and add some alfalfa to her diet. The worry with minis is usually them being too FAT, not thin, so I usually say not to feed alfalfa or high fat feeds, but it appears she is going to need it. Get her a flake of alfalfa (or 3-4 lbs of alfalfa pellets) a day if you can, and as much of the other hay you can stuff into her. A higher fat, higher protein feed for a short time might be a good idea as well.