All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
First of all, we congratulate you on the blessing of embracing Islam, the true religion and the straight path by which a person achieves happiness in this world and the Hereafter. We ask Allaah to make you and us firm in it until we die. We appreciate your eagerness to raise your children on the right path. May Allaah reward you and enable you to achieve what you wish for your children. Allaah listens to our supplications and He responds to them.
Our best advice is that you supplicate on behalf of all your children, the young ones and the adults; all guidance comes from Allaah and the hearts of His slaves are in His hands and He turns them the way He wills. Allaah says (what means): {Indeed, you do not guide whom you like, but Allaah guides whom He wills. And He is most knowing of the [rightly] guided.} [Quran 28:56]
We mentioned some guidelines for dealing with stubborn children in Fatwa 237535, so please refer to it.
We emphasize here an important matter, especially concerning your adult children: Besides being firm with them, one has to be patient, gentle and compassionate and try to show them the virtues of Islam when dealing with them. You may also have them listen to the lectures of Shaykh Ahmed Deedat, which are truly effective.
Among the things educators advise concerning stubborn children is to respond to their poor behavior with extreme calmness, as aggravation can cause mistaken reactions, which can in turn result in bad consequences. In addition, they advise ignoring some stubbornness because addressing every action reinforces such negative behavior, so children insist on repeating it even after being punished or insulted because of it. This occurs often. They also remind us that we should encourage positive behavior and praise and reward children for it, and so on.
Finally, it is very important that you occupy the time of your adult children with what benefits them, for example by enrolling them in centers for developing their athletic skills, academic skills and the like. At the same time, you should work on instilling the Islamic faith in the hearts of your young children. If the Islamic creed is rooted in them, they will denounce what their adult brothers do. Their pure souls accept all good guidance; indeed, instruction in youth is like etching in stone. For more benefit, kindly refer to Fatwa 133848.
Allaah Knows best.