The Rebbe's guidance to parents whose son or daughter is drifting from Torah and mitzvos:
Parents have more influence than they think. Many parents assume their efforts are in vain, or are embarrassed to raise these matters with their children, and so lose much of the influence they could have had. In truth they can accomplish far more than they imagine — perhaps not everything, but a great deal (IK 1063, 1519). True love for a child shows itself in matters of the soul no less than of the body; draw each child closer in the way and with the words best suited to him (IK 777). Persistent pleading never comes back empty, least of all from a parent (IK 1735).
Speak gently, and speak again. Bitterness alone does not help. "The words of the wise are heard when spoken gently": say it once, twice and more, with kind words and requests, and it will have an effect — a little at first, then more and more (IK 1293, 1377). Ask and explain rather than argue, and make clear that living as a Jew is not a favor to G-d but the way to a good and happy life (IK 2348). In almost every case a pleasant approach achieves more than the opposite (IK 7610).
Friends often reach a child better than parents can. Find friends and respected people the child will listen to, and ask them to speak with him repeatedly and from the heart — ideally without it being apparent that the parents sent them (IK 4345, 7344, 7629, 8742, 9199, 10579, 10944). Keep the connection with the child, without condoning the behavior (IK 9219).
Your own example matters most. Children often stray because their parents are not wholehearted in their own observance; strengthening your own daily conduct is part of the answer (IK 7610, 7629, 10579). Don't settle for compromises in your home and surroundings (IK 4345). A child's threats should not sway you; explain calmly and with feeling where the path he is choosing leads (IK 7629).
A child involved with someone not Jewish. Act promptly but discreetly: involve trusted people, quietly help the child meet suitable Jewish partners, keep the home's spiritual safeguards in place, and make your values clear without causing a break (IK 1771, 6197, 8745, 8970, 10501). Even after the most painful news, take practical steps and don't give up (IK 5758).
Practical spiritual steps. Rather than fasting, give extra tzedakah; have tefillin and mezuzos checked, strengthen family purity and the mother's custom of charity before candle-lighting (IK 5758, 7344, 8742). Pray — but also do what can be done by natural means, with trust and joy (IK 8538).
See also: Marriage – Intermarriage (marrying a non-Jew); Parenting – Discipline, warmth and setting an example.