The Holy Prophet of Islam (peace be upon him and his progeny) said:
إِعْدِلُوا بَيْنَ أَوْلادِكُمْ كَما تُحِبُّونَ أَنْ يَعْدِلُوا بَيْنَكُمْ
Observe justice among your children just as you would like that they render justice among you.1
One of the capital mistakes is discrimination in treatment of children. Some people favour their older and so called senior child and others favor their younger child. Sometimes they exceed the bounds and reserve all of their kindness, property and affections for one of them, and deprive the others totally.
This provokes the fire of malice and envy in the hearts of the ones left out and in time this will develop into enmity among them and vindictiveness towards their parents and they may even seek vengeance in the society.