This is not an easy question to answer and every situation will be different given the personalities involved. Even so, it is possible to provide a few suggestions that can likely be utilized in many circumstances.
In his book-length interview My Brother, The Pope, the Reverend Monsignor Georg Ratzinger, after remembering the Christmas traditions of his family, said (with my emphases):
In our family, though, it was not only Christmas that was marked by the deep faith of our parents and the religious customs of our homeland. From our parents we learned what it means to have a firm grasp of faith in God. Every day we prayed together, and in fact before and after each meal (we ate our breakfast, dinner, and supper together). The main prayer time was after the midday dinner, when the particular concerns of the family were expressed. Part of it was the prayer to Saint Dismas, the "good theif", a former criminal who was crucified together with Jesus on Mount Calvary, repented on the cross, and begged the Lord for mercy. We prayed to him, the patron of repentant thieves, to protect Father [a policeman] from professional troubles.If we think Monsignor Ratzinger is a bit harsh in his assessment or old fashioned, we simply deceive ourselves. Now, then, to Steve's excellent question. I don't quite know the best way to go about it, so I'll simply make a list of suggestions, in no particular order:
....When we were children, our parents also put us to bed and prayed our evening prayers with us. They used a special form of blessing and repeated it three times....
This piety, which was lived and put into practice, defined our whole life... [I]t was imparted to us children in the cradle, so to speak, and we remained faithful to it throughout our lives.
I am convinced that the lack of this traditional piety in many families is the reason why there are too few priestly vocations today. Many people in our time practice a form of atheism rather than the Christian faith. In some respects, they may maintain a vestigial religiosity; perhaps they still go to Mass on the major feast days, but this rudimentary faith long ago ceased to permeate their lives, and it has no bearing on their everyday routine. It starts with sitting down at table and beginning a meal without even thinking about prayer, and it ends with no longer coming to church regularly on Sundays. Thus, an almost pagan way of life has taken root. If there are no religious practies even in family life, then this has an effect on all the rest of human life (45-47).
- Children must see in their parents an authentic love of the faith and a desire to be faithful in every aspect of their lives, and in such a discipleship the children must sense the joy that comes from following Christ.
- Children should see their parents in prayer, both before the tabernacle and at home. Children learn from imitation. If they see their parents pray, they, too, will learn to pray. This means that parents must pray; this has to come first.
- Set up a family altar, a special table covered with a cloth in a prominent place in the home. On the table, place a Bible, a book of the saints, a book of prayers, a candle, a statue of a patron saint, a rosary, etc. On the wall above the table hang a crucifix. I always like the San Damiano crucifix because it also tells the story of the Lord's Passion and can be used throughout the year as a teaching device. Use this family altar each day and try to do so at the same time of day. Build it into the family schedule. Everything else is secondary; God must come first. Always.
- Never water down the faith or the teachings of the Church, especially if they are in high school. Despite the temptation, never pander the faith to your children. They can handle it and they can wrestle with it. Young people today simply want to know the truth; speak it to them plainly.
- If you have to give a simplistic answer to a question when your child is young (for good reason) be certain to expound that answer as your child grows up. Do not leave them with a second grade understanding of, say, Holy Communion or Confession when they're 16.
- If you do not know the answer to your child's question about the faith, do not say, "I don't know; it's a mystery" (unless it genuinely is a mystery, but even then say something about it. Remember: the Holy Trinity is the only part of Christian faith that cannot be known by reason; in the end, it truly is a mystery. Everything else can be explained reasonably, even if only in part). If you don't know the answer, simply say, "That's a great question and I don't know the answer to it, but I'll be glad to find the answer for you." Consult your catechism, your pastor, a knowledgable friend, and scour the Internet if you have to. Just find the answer. And do so as quickly as you can.
- Always to seek to show not only the beauty and simplicity of the faith, but also the reasonableness of the faith.
- Study the history of the Church - especially if your children are in high school - to be able to explain what really happened with the Crusades, the Inquisition, Galileo, etc. Common myths should be debunked.
- Pray the rosary together at least once a week.
- Go to confession at least once a month (more often if you need it) and bring with your children with you (to the church, anyway). Let them see you enter the confessional and return to the pew to offer a prayer of thanksgiving. Encourage them to go to confession, too.
- Help your chilren examine their consciences from time to time, and provide them with age-appropriate guides.
- Give your children a religious book at Christmas, Easter and other major celebrations.
- Celebrate the day of your children's baptism with more festivity than their birthday.
- Have a special dinner each year on their "name day," the feast day of the patron saint.
- Name your children after saints and teach them their stories.
- Talk about the saint of the day and the daily Mass readings at supper.
- Talk about the Pope's travels or his Angelus addresses together.
- Pray the Stations of the Cross together each Friday.
- Go to Eucharistic adoration together as a family.
- We running your daily errands, purposefully drive by your church and stop in for a moment of prayer together. You know, just to say "Hi" to Jesus.
- Share your faith struggles and questions with your children. It's good for them to know that you wrestle with the same struggles and questions as they do, but be sure to reinforce your faith in the midst of doubts. Teach the beauty of the words, "Lord, I believe, help my unbelief."
- Be sure to show your concern for your children's faith and eternal salvation.
- Never answer the question, "Why do we have to go to Mass today" saying, "Because I said so."