"It is wrong" ought to be enough, and the less children talk of mortal sin the better--to talk of it, to discuss with them whether this or that is a mortal sin, accustoms them to the idea.

When they know well the conditions which make a sin grave without illustrations by example which are likely to obscure the subject rather than clear it up, when their ideas of right and duty and obligation are clear, when "I ought" has a real meaning for them, we shall have a stronger type of character than that which is formed on detailed considerations of different degrees of guilt.
On the other hand it is possible to confuse and torment children by stories of the exquisite delicacy of the consciences of the saints, as St.Aloysius, setting before them a standard that is beyond their comprehension or their degree of grace, and making them miserable because they cannot conform to it.
It is a great safeguard against sin to realize that duty must be done, at any cost, and that Christianity means self-denial and taking up the cross.
4.

Eight thoughts of the four last things.

True thoughts of death are not hard for children to grasp, to their unspoiled faith it is a simple and joyful thing to go to God.

Later on the dreary pageantry and the averted face of the world from that which is indeed its doom obscure the Christian idea, and the mind slips back to pagan grief, as if there were no life to come.
Eight thoughts of judgment are not so hard to give if the teaching is sincere and simple, free from exaggerations and phantoms of dread, and on the other hand clear from an incredulous protest against God's holding man responsible for his acts.
But to give right thoughts of hell and heaven taxes the best resources of those who wish to lay foundations well, for they are to be foundations for life, and the two lessons belong together, corner-stones of the building, to stand in view as long as it shall stand and never to be forgotten.
The two lessons belong together as the final destiny of man, fixed by his own act, _this_ or _that_.