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Dogma - “Regarding children, indeed, because of danger of death, which can often take place, since no help can be brought to them by another remedy than through the sacrament of baptism, through which they are snatched from the domination of the devil and adopted among the sons of God, [the sacrosanct Roman Church] advises that holy baptism ought not to be deferred for forty or eighty days, ... but it should be conferred as soon as it can be done conveniently…” -Pope Eugene IV, Council of Florence, Cantate Domino, February 4, 1442

["Likewise, whoever says that those children who depart out of this life without partaking of that Sacrament (Baptism) are alive in Christ, certainly contradicts the apostolic declaration and condemns the universal Church, in which it is the practice to lose no time and run in haste to administer Baptism to infant children, because it is believed as an indubitable truth, that otherwise they cannot be made alive in Christ." -Saint Augustine, Father and Doctor of the Church, Epistle 167, AD 415

Nor were the Holy Innocents before martyrdom. Some were not even circuмcised (cf. St. Robert Bellarmine De Sacramento Baptismi p. 166, first full ¶); thus, they were justified solely by martyrdom.

One needn't be of the age of reason to be martyred. See Pope Benedict XIV's bull Beatus Andreas for several examples of young martyrs.

How do you explain the Holy Innocents, then?