Pope St. Zosimus, The Council of Carthage, Canon on Sin and Grace, 417 A.D.- “It has been decided likewise that if anyone says that for this reason the Lord said: ‘In my Father’s house there are many mansions’ [John 14:2]: that it might be understood that in the kingdom of heaven there will be some middle place or some place anywhere where the blessed infants live who departed from this life without baptism, without which they cannot enter into the kingdom of heaven, which is life eternal, let him be anathema.”
Interesting quote, but it speaks to a middle place WITHIN the Kingdom, and does not rule out some middle place (
Limbo Infantium) outside the Kingdom proper. We understand the Kingdom as entailing the Beatific Vision and adopted into the supernatural family of the Holy Trinity.