Yes you need to get them out for exercise as soon as possible - weather allowing. It is not good for mares to stay confined after foaling - unless for medical reasons - as they can get very stressed which is not good for their milk supply! Also the new baby needs to be able to run around to help its digestion and to expand its lungs/strengthen its muscles.
I would keep your younger mare inside for a few days while Momma and baby are enjoying their outside time - Momma is already telling you that she doesn't want the young girl anywhere near her new baby, so I wouldn't try with them next door to each other with just a fence inbetween them for a few days yet - Momma rushing to the fence to 'chase off' the youngster, and possibly kicking out could get the foal unintentionally injured.
Once Momma has settled down a bit, then try your young girl next to her with a safe (foal safe as well) fence between them. You may have to keep them like this for quite a long while depending upon what your mare thinks - her behaviour will tell you when the time is right to try putting them all together. How soon can you get Momma and baby out into a larger paddock? Once baby is rushing around, small areas are not so good as he will be having to do too many stops and starts with his young, immature muscles, he needs the space to be able to run in large circles to build up his strength and to expand his lung capacity.
Of course, as Renee says, things are governed by the amount of space available and I dont know your set up, but the rule of thumb is to always give your animals (especially the youngsters) as much space as is possible.
Oh and dont forget the camera when you first let them out!!