We have a little "creep" feeder in the foaling stall for the foals. There is a bar where the foal can get under, but Mom can't. At about ten days old we start putting a few grains of pelletized feed (Progressive Grass Balancer) in the bowl for the foal. The pellets break down easily and are digestible. They love going in their little corner of the stall, sometimes just to play, and others to nuzzle the grain. By about a month of age, they are eating a few grains, and by six weeks eating some every day, along with grass in the pasture, and a tiny bit of hay. By three-four months of age they clean up their little bowl, perhaps up to 1-2 cups per day. They also use the water bucket in the stall because it is positioned low. We never have any trouble weaning, because they are already eating and growing well.
If you will check, your filly has teeth by now, and knows how to use them! So they are ready for independent eating........not just mother's milk.