Why babies can receive communion in Eastern Churches

Even Roman Catholics practiced infant communion for many centuries, but changed its practice for various reasons.

Roman Catholics are accustomed to seeing children around the age of 7 receive holy communion for the first time. Yet, that practice is relatively new in the Catholic Church and until the 13th century, both East and West distributed communion to infants.

St. Augustine wrote about this practice in one of his sermons.

They’re infants, but they receive his sacraments. They are infants, but they share in his table, in order to have life in themselves.”
Augustine, Sermon 174, 7

The Catholic Encyclopediaalso affirms this historical practice.

It is now well established that in the early days of Christianity it was not uncommon for infants to receive Communion immediately after they were baptized. Among others St. Cyprian (De Lapsis 25) makes reference to the practice. In the East the custom was pretty universal, and even to this day exists in some places, but in the West infant Communion was not so general.

Eventually the Council of Trent curtailed this practice in the Western Church, explaining the need for children to understand the difference between ordinary bread and the consecrated host.

For these are not able to distinguish the Holy Eucharist from common and ordinary bread and cannot bring with them to this Sacrament piety and devotion. Furthermore (to extend the precept to them) would appear inconsistent with the ordinance of our Lord, for He said: Take and eat words which cannot apply to infants, who are evidently incapable of taking and eating.
Catechism of Trent

Sacraments of Initiation

Yet, while the Roman Church moved away from infant communion, it was done so for pastoral reasons and not theological ones. It has been long held that the Eucharist is the final sacrament in Christian initiation, as the Catechism of the Catholic Church explains.

The holy Eucharist completes Christian initiation. Those who have been raised to the dignity of the royal priesthood by Baptism and configured more deeply to Christ by Confirmation participate with the whole community in the Lord’s own sacrifice by means of the Eucharist.
CCC 1322

In the Eastern Church, the sacraments of initiation (baptism, confirmation and the Eucharist) are celebrated together at whatever age a person enters the Church. In most cases, this occurs shortly after birth as an infant.

The Code of Eastern Canon Law affirms this idea and still permits infant communion for this reason.

Sacramental initiation into the Mystery of Salvation is perfected through the reception of the Most Holy Eucharist. Therefore let it be administered as soon as possible after baptism and chrismation with the Sacred Myron, according to the discipline proper to each Church
Canon 697

In the end, infant communion is still practiced in Eastern Churches in order to preserve the sacraments of initiation and to provide even little children the special graces they need. The Roman Church decided to abandon the practice in favor of conscious participation in the sacrament. Either way, both practices are valid and part of the rich tradition of the Catholic Church.