In the previous post, Faith, Intention and Sacramental Reception of the Eucharist, I spoke about the necessity of an intention to receive the sacrament of the Eucharist in order to enter into the sacramental union with Christ specific to this sacrament (rather than merely have Christ within one's body, just as a ciborium does). This raises a question about infants receiving Communion, as is customary in Eastern rites, both Catholic and Orthodox. Can they be said to receive sacramentally, and to receive the grace of the Sacrament?
Thomas Aquinas's position on this is not entirely clear. Though he knows about the Eastern practice of giving the Eucharist to infants, he argues that this is unfitting. The Eucharist should be given only to those who have or have had devotion to it (Summa Theologiae, III q. 80, a. 9). It can be given to those who have lost the use of reason on the condition that they previously had devotion to the Sacrament, but should never be given to those who have never attained the use of reason. It is not clear, however, that Thomas Aquinas considers these persons incapable of thereby being spiritually and sacramentally united to Christ through the Sacrament of his Body and Blood. It may even be more probable that he simply considers it unfitting to the dignity of the Sacrament to give it to those who have shown no actual devotion to it.
Two points should be made in regard to the question: first, the degree to which this was and is practice of the Church makes it virtually certain that the Sacrament can in fact be sacramentally received and fruitful for infants too young to know what they are doing. (St. Thomas was not aware of how wide-spread the practice had been). It was at one time almost universal practice to give Holy Communion to infants, at least on certain occasions, such as at baptism or when in danger of death–the latter practice was seemingly based, in some cases, upon a very literal reading of John 6:53 ,"unless you eat the flesh of the Son of man and drink his blood, you have no life in you"; but even if partially based on a misunderstanding, it manifests the conviction that infant reception of the Eucharist was not merely understood as a sign for the adults, but as fruitful for the infants themselves. Again, it has remained and is common practice in multiple rites of the Church. To maintain that the Church has been and is all this time practicing a kind of abuse of the Blessed Sacrament by giving it to those incapable of receiving it, as though they were capable of receiving it, is not a very sound position.
Secondly, when infants receive the Eucharist in an ecclesial and liturgical context, the theological principle that those who are incapable of making an act of faith on their own receive the sacrament and grace in virtue of the faith of others, well established in the case of baptism and confirmation, seems just as applicable. Infants who receive Communion from those who intend to give the Sacrament to them, receive Christ sacramentally and receive grace from him.
For some more information on the practice of Infant Communion, an article by Charles Crawford, Infant Communion: Past Tradition and Present Practice (PDF), is available online.