A horse's intestines evolved to process grasses, hays, twigs, and even some bark. It has never been designed to process high levels of sugars and starches and many of the grains that are included in a sweet feed.
Just because it was common practice 20 years ago when you were breeding halter horses to coop a foal up in a stall for ~20 hours a day and stuff their gut full of sugars and starches and anything else that worked to make them grow faster and look more "mature" by the time they were a yearling doesn't mean that it was healthy or safe for them.
OP, if you want to creep feed the foal, then just give the mare plenty of hay at a level where the foal can get his/her head in too. If the mare or foal seem to need something more, then I would add something that was actually nutritionally designed for lactating mares and/or foals.