The best explanation for how infant communion was abolished in the Roman rite is herein explained by J.D.C. Fisher Sadly, the title of his book had to include “A Study in the Disintegration of the Primitive Rite of Initiation” since, let’s be frank, that is precisely what occurred. Here goes it:
“In the eleventh century, however, doubts began to arise about the propriety of communicating infants and sick persons in consequence of a growing scrupulosity regarding the consecrated elements, itself the result of the gradual victory of Realism over Symbolism… In these circumstances the Church began to feel uneasy about the communion of persons who might not be able to swallow the hosts… The first step taken to obviate the grave risk involved in communicating infants was to give them communion sub specie sanguinis (under the species of blood) only… But the doctrine of Realism led in time also to a growing disinclination to permit any of the laity ever to communicate from the chalice, a development which discouraged and eventually brought to an end the practice of communicating newly baptized infants sub specie sanguinis. It became increasingly common in the West during the thirteenth century for the laity to be communicated sub specie corporis (under the species of the body) only… Consequently in view of the reluctance which we have observed to give the consecreated host to infants, in many places infants ceased to be communicated at all… Infant communion, it seems, was not finally abolished in the West until the Council of Trent declared that infants lacking the use of reason were under no obligation to receive the sacramental communion of the Eucharist, since, having been regenerated through the laver of baptism and incorporated into Christ, they could not in infancy lose the grace of sons of God then received. But the Council was hard put to it to explain away the practice of the past without appearing to condemn it. Antiquity, it said, was not to be condemned if it maintained that custom (i.e. of giving communion to infants) in certain places for a time — an extraordinary statement in view of the fact that ALL CHURCHES IN THE WEST admitted infants to communion until the twelfth century, and this practice had been generally approved for a thousand years. Nor is this all: for the Council of Trent went on to allege that the holy fathers of antiquity found an adequate reason for what they did in the situation of that time, and without treating communion as necessary to the salvation of infants, a claim which is not in accordance with the facts.”
J.D.C. Fisher, Christian Initiation: Baptism in the Medieval West: A Study in the Disintegration of the Primitive Rite of Initiation (London: SPCK, 1965), 100-107; Alcuin Club Collections No. XLVII
Everything above seems to be entirely accurate… save for the last sentence. While it is true certain scholastic thinkers thought that it was necessary to consume the Body and Blood of Christ (via at least one kind) was absolutely necessary for salvation such that, without doing so, one is consigned to the flames of hell, this doesn’t appear to be well supported by the Church Fathers. If anyone can point me in that direction, I’d like to see what you can point to. I’ve written briefly on that matter (Baptized but no Eucharist? Heaven or Hell). Beside this, I find Fisher’s explanation for the abolition of infant communion to be sufficient to explain the change.