As soon as wrongful thoughts come into your heart, dash them against Christ and disclose them to your spiritual guide (1 Peter 5:5-7).
I don't have a "spiritual guide" and I'm not sure how to explain that one to my children except to say that we should express all our thoughts to Jesus in our personal prayer and confess our sins to the priest (when they are old enough)*. That being said, I have had a spiritual director in the past and found it very helpful and, coincidentally, I've been thinking of getting another one just recently. Maybe this is confirmation. A spiritual director is someone trained in spiritual direction who is there help and to guide a person on his or her personal spiritual journey.
Stopping our wrongful thoughts, on the other hand, is easier to explain to a child. It is a great habit of virtue. Often our thoughts that are not holy turn to words and even to actions. That is the reason we must "dash them on Christ (our rock)" and turn our thoughts to good and to prayer. It's a constant battle but fighting "wrongful thoughts" is worth the reward: peace.
*Confession, also known as Reconciliation, is a sacrament in the Catholic Church. Some of our seven Sacraments are: Confession, Baptism, Eucharist and Marriage. Sacraments are not our action but God's. They are a gift to us that we receive. Sacraments are spiritual food, instituted by Christ himself, that help us get to our final destination: eternal life with God.