Objection 1. It would seem that suffrages avail the children who are in limbo. For they are not detained there except for another's sin. Therefore it is most becoming that they should be assisted by the suffrages of others.
Objection 2. Further, in the text (Sent. iv, D, 45) the words of Augustine (Enchiridion cx) are quoted: "The suffrages of the Church obtain forgiveness for those who are not very bad." Now children are not reckoned among those who are very bad, since their punishment is very light. Therefore the suffrages of the Church avail them.
On the contrary, The text (Sent. iv, D, 45) quotes Augustine as saying (Serm. xxxii, De Verb Ap.) that "suffrages avail not those who have departed hence without the faith that works by love." Now the children departed thus. Therefore suffrages avail them not.
I answer that, Unbaptized children are not detained in limbo save because they lack the state of grace. Hence, since the state of the dead cannot be changed by the works of the living, especially as regards the merit of the essential reward or punishment, the suffrages of the living cannot profit the children in limbo.
Reply to Objection 1. Although original sin is such that one person can be assisted by another on its account, nevertheless the souls of the children in limbo are in such a state that they cannot be assisted, because after this life there is no time for obtaining grace.
Reply to Objection 2. Augustine is speaking of those who are not very bad, but have been baptized. This is clear from what precedes: "Since these sacrifices, whether of the altar or of any alms whatsoever are offered for those who have been baptized," etc.
Source: Summa Theologica (New Advent)