She's only 8 days old. Truthfully, I'd be giving her full access to hay, but I wouldn't be feeding her much of anything else. If momma is getting the same thing as far as grain/feed goes, the baby will likely take a few bites while mom eats.
Feeding a foal too much can cause some very serious issues with their growth and metabolism, predisposing them to arthritis and several other health issues.
It takes a baby a little bit of time to build up the muscle and fat to cover their ribs, you don't want to push them to get too fat too quickly. Actually, on growing foals, it's better to see a rib or two than have them be overweight.