Some notes.
Babies which are still dependent on milk get really ill when you change their milk abruptly.
If this baby is young enough to require nothing but milk and hasn't already established a happy routine of drinking horse-milk, this baby is going to get seriously ill, seriously quickly. It will get an upset stomach, probably resulting in D&V, and will be dehydrated and screaming before you know it.
Horses don't like being milked if they're not already used to it. Unless your MC is an incredible horsewoman as well as a nanny she's going to struggle to do this.
Colts are male baby horses, fillies are female baby horses, foals are all baby horses.
Most babies can last a few hours between feeds, especially if they're formula-fed.
Babies can catch diseases from raw milk, which is what this would be: it's really not a good idea. Better to wait for the parents to return even if they are late.
Strictly speaking, you could feed many babies from the milk from one mare if the mare were willing to be milked, which few would be if they haven't been milked before. Milk is made according to demand. So the more milk you take, the more she makes.
If the mare has only just had a foal in the last couple of days she'll only be making colostrum, not milk, which is only made in relatively small amounts and there won't necessarily be enough for more than her own foal--and it's likely that the colostrum will be even more troublesome for the babies' tummies.
An experienced nanny would probably not go near horse-milk if there was no baby milk available, because there are so many problems associated with it. Sugar water might be a better solution to hungry babies; it would be sterile and would feed their hunger even if it would also carry the risk of upset tummies later on down the road.