You state that paragraph 1457 states this without qualification. I don't think this is precisely true. The paragraph states in full:
According to the Church's command, "after having attained the age of discretion, each of the faithful is bound by an obligation faithfully to confess serious sins at least once a year." [quoting canon 914] Anyone who is aware of having committed a mortal sin must not receive Holy Communion, even if he experiences deep contrition, without having first received sacramental absolution, unless he has a grave reason for receiving Communion and there is no possibility of going to confession. Children must go to the sacrament of Penance before receiving Holy Communion for the first time.
Clearly, the point of going to Confession before First Holy Communion is to ensure that the communicant is cleansed of sin at reception of the Eucharist. And as you point out, infants (and in the Latin Church, children under age 7) are incapable of committing sin. The statement in paragraph 1457 therefore can only apply when one's First Communion takes place after the beginning of the age of reason. And in fact, we see this in the CCEO (the Codex Canonum Ecclesiarum Orientalium, or Code of Canons of the Eastern Churches). The CCEO discusses requirements surrounding the Divine Eucharist in its Title XVI, Chapter 3 (canons 698–717). Only canon 710 discusses this situation:
Circa infantium in Divina Eucharistia participationem post baptismum et chrismationem sancti myri serventur opportunis adhibitis cautelis praescripta librorum liturgicorum propriae Ecclesiae sui iuris.
Concerning the participation of infants in the Divine Eucharist after baptism and chrismation with holy myron, the prescripts of the liturgical books of the relevant Church sui iuris are to be observed with the due and appropriate cautions.
There is nothing here about a minimum age (as there is in the Latin Rite Code of Canon Laws, canon 914), nor about the need to make a confession first.
Why then would the Catechism not make a specific statement that this applies only to the Latin Rite Church? A few reasons I can think of:
- Most Catholics (about 98.6%) are Latin Rite Catholics; offering Eucharist to infants post-baptism and chrismation is decidedly a minority practice.
- The practice is purely a ritual one, like the other differences between the Eastern and Latin rites; since the Catechism is a teaching document, talking about doctrine and theology, the distinction of practices isn't relevant.
- The Catechism of the Catholic Church is a general catechism. By that, as it says itself, it is intended "primarily for those responsible for catechesis: first of all the bishops, as teachers of the faith and pastors of the Church." (paragraph 12) The bishops (eparchs in the Eastern Churches), of course, will understand and be able to explain why Confession cannot be required in the case of infants; and the particular catechisms meant for specific groups of the laity will no doubt reflect this teaching.