Has a child of seven sufficient reason?

As a rule, yes. The law of the Church to receive Holy Communion once a year obliges all Catholics who have come to the use of reason, and this law begins to oblige from about the age of seven. The average child of seven certainly has enough sense to realize that the reception of the Holy Eucharist is a religious act. It can know who Our Lord is, and the fact that He is present in the Blessed Sacrament. Such a child is quite capable of approaching with sincere faith and devotion

Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty