You need three things: the Baltimore Catechism, loving parental guidance and a wise confessor.
The BC is directed to children and is very clear on the nature of sin and the necessity of the Sacrament of Confession. Read it with your child (don’t just make them read it) so that you can flesh out the explantions and answer questions. That will give them the solid building blocks. Don’t rely on this, however as the child may either find it too abstract or develop scruples.
Find a wise confessor who will hear the child’s confession and give them some guidance in developing perspective (make sure you have this in place before you teach them about sin and the need for Confession). The child may start out confessing every little thing, but he will learn by experience through gentle guidance what is and is not a mortal sin.