I don't know what kind of mare & foal feed you're giving, nor do you give the weight your mare SHOULD be at, so I'm going to just generalize with Purina Omolene 300.
For a 1000 lb LACTATING mare she should be getting 12.5 lbs/day, so 6.25 lbs 2X/day or 4.2 lbs 3X/day. She should also be getting a minimum of 12 lbs of hay/day, but I prefer to give free choice grass hay. If you're feed directions on the bag are similar to the Omolene's then you should increase her feed by 1 lb per feeding, so from 6 lbs to 8 lbs/day for 1 week, then go to 10 lbs/day for 1 week and then 12.5 lbs from there on. If she needs to be bigger, then you increase proportionately. For me, I generally feed for a 1200 lb mare until I'm comfortable with how the mare I wanted to be 1000 lbs looks and then I back down to that amount of feed for maintenance.
You don't say how old or what size the foal is, so I can't really guess how much feed to give them in a creeper.