We have a month old foal and her mom which is the head mare at what age can they be returned to the herd?

Poco is 5 years old and head mare her girl was born March 22 2008. we have two mini horse and one three year old standardbred filly and a 3 year old throughbred stallion. ALso a holstein cow due at the end of April this year. They were all together before and they were all their in the feild when the little filly was born and no problems. We moved mom and baby to their own penned in area about six hours after birth. They all talk to each other and they hang out were they can see Poco the mom. the baby is kicking up and poco is the greatest mom ever. So whould it be safe to turn them lose with the herd. We of course, would watch them when we do. When would be a safe age. We have had people say any time some say a few more months. What do you all say.