Upvote:4
Ordinarily, people (children and adults) simply tell the priest in confession what they did. The priest will determine whether the sin was mortal or venial (or perhaps no sin at all), possibly after asking the penitent some questions to get a clearer idea of the penitent's intention and state of mind, or of the circumstances surrounding the sin. The priest might but need not tell the penitent explicitly whether he considers the sin mortal or venial.
Upvote:4
Your question has more to do with culpability than it has to do with anything else, and that is a sticky subject.
For a sin to be mortal it must:
So if a three year old fires a gun and kills someone, that would be a grave matter, the act might have had consent of the will, but does the child understand? Probably not (not impossible, but probably not).
I believe I read this in a commentary on the Code of Canon Law (but maybe not), but the loose standard for full culpability is a rule of sevens.
Obviously there are exceptions, but those are guidelines.