Some time ago a question was raised about the baptism of blood, and how this is applicable to infants. Normally a martyr is someone who witnesses to Christ in the ultimate circumstance, when it means giving up his life for Christ. But infants don't know anything about Christ, and thus seemingly can't witness either to the person of Christ or to Christian truth. In what sense, then, is the shedding of their blood a baptism?
I said then that the intention of the one who kills an infant, an intention directed against Christ, is reason enough to see the infant as standing in the place of Christ, and consequently, likened unto Christ in his suffering, and thus receiving the grace of Christ. This intention against Christ is most direct in cases such as that of St. Simon of Trent, a two year old child killed out of hatred for Christ.
I also suggested that "being unjustly put to death" might, from God's point of view, count as such conformity to Christ, and thus be a way to receiving grace in the case of an infant, whose will does not place any impediment to grace.
The case of St. Coloman provides some, if only weak evidence for this. He was an Irish monk who was making a pilgrimage to Jerusalem, and in Austria detained under suspicion of being a spy. Unable to explain his purpose, since he could not speak German, he was tortured and killed. He is celebrated as a martyr, despite the fact that he was put to death by Catholics not for religious, but for quasi-military reasons. Though he found his death while making a pilgrimage, the final or decisive factor (a kind of formal cause) constituting his martyrdom seems to be being unjustly killed, or being unjustly killed despite his religious intent (rather than because of it), since (at least according to one account I've seen) he showed his cross and pointed East to indicate his intentions, but wasn't believed.