Negative consequences of harshness
Islam forbids parents to discipline their children with harshness. Moreover, it only brings negative consequences. Children do not learn by hitting them hard, but rather when they are encouraged and treated with respect.
Children test situations to see where the boundaries are. It is then up to the parent to deal with this properly and to discipline them. This should not happen by hitting as a first resort, but rather by patiently explaining it to them first.
Consequence 1: Resentment and hatred
Children who are severely beaten no longer trust their parents. They go out alone and make friends where they feel safe. The child will grow up and will slowly move away from the abusive parent until he cuts the family bond.
١٥٢ - حَدَّثَنَا الْحُسَيْنُ بْنُ الْحَسَنِ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ بَكْرٍ السَّهْمِيُّ، حَدَّثَنِي صَاحِبٌ لَنَا يُكَنَّى أَبَا وَاثِلَةَ , أَنَّ مُعَاوِيَةَ دَخَلَتْهُ مَوْجِدَةٌ عَلَى ابْنِهِ يَزِيدَ فَأَرِقَ لِذَلِكَ لَيْلَتَهُ فَلَمَّا أَصْبَحَ بَعَثَ إِلَى الْأَحْنَفِ بْنِ قَيْسٍ فَأَتَاهُ فَلَمَّا دَخَلَ عَلَيْهِ قَالَ لَهُ: يَا أَبَا بَحْرٍ، كَيْفَ رِضَاكَ عَلَى وَلَدِكَ؟ وَمَا تَقُولُ فِي الْوَلَدِ؟ قَالَ: فَقُلْتُ فِي نَفْسِي: مَا سَأَلَنِي أَمِيرُ الْمُؤْمِنِينَ عَنْ هَذِهِ إِلَّا لِمَوْجِدَةٍ دَخَلَتْهُ عَلَى يَزِيدَ فَحَضَرَنِي كَلَامٌ لَوْ كُنْتُ زَوَّقْتُ فِيهِ سَنَةً لَكُنْتُ قَدْ أَجَدْتُ فَقُلْتُ: «يَا أَمِيرَ الْمُؤْمِنِينَ هُمْ ثِمَارُ قُلُوبِنَا وَعِمَادُ ظُهُورِنَا وَنَحْنُ لَهُمْ أَرْضٌ ذَلِيلَةٌ وَسَمَاءٌ ظَلِيلَةٌ وَبِهِمْ نَصُولُ إِلَى كُلِّ جَلِيلَةٍ فَإِنْ غَضِبُوا يَا أَمِيرَ الْمُؤْمِنِينَ فَأَرْضِهِمْ وَإِنْ طَلَبُوكَ فَأَعْطِهِمْ يُمْحِضُوكَ وُدَّهُمْ وَيَلْطُفُونَ ⦗٣٠٩⦘ جُهْدَهُمْ وَلَا تَكُنْ عَلَيْهِمْ ثَقِلًا لَا تُعْطِيهِمْ إِلَّا نَزْرًا فَيَمَلُّوا حَيَاتَكَ وَيَكْرَهُوا قُرْبَكَ»
Mu'awiyyah was angry with his son Yazid, so Ahnaf ibn Qays told him:
"O leader of the believers, they (children) are the fruits of our hearts and the pillars of our presence, and we are an easy land and a cool air for them, and through them we attain every greatness. When they are angry, forgive them, and if they ask for something from you, give them, for it makes them grateful and gentle towards you. Do not be heavy on them, nor give them just a little, so that they do not resent your life and your nearness."
[Al-'Iyal 1/308 from Ibn Abi Dunyah]
Consequence 2: Oppression and suppression
As a parent, you are not supposed to be bossy over your children and want complete control over them. When you do this, you will notice that the child will keep things secret from you.
Do not try to suppress your children by taking over their lives. They also have their dreams and wishes that they want to fulfill. As a parent, you should encourage them to do so when this is halal.
It is oppression and abuse to beat children harshly. Do not go beyond the limits prescribed by Allah, otherwise this will be oppression and transgression.
حدثنا هارون بن عمر الدمشقي قال، حدثنا عبد الله بن كريم قال، حدثنا أبو الفتح، عن حبيب بن أبي مرزوق قال: دخل عثمان بن عفان رضي الله عنه على غلام له يعلف ناقة، فرأى في علفها ما كره، فأخذ بأذن غلامه فعركها، ثم ندم فقال لغلامه:
اقتص. فأبى الغلام، فلم يدعه حتى أخذ بأذنه فجعل يعركها، فقال له عثمان: شد حتى ظن أنه قد بلغ منه مثل ما بلغ منه، ثم قال عثمان رضي الله عنه: واها لقصاص قبل قصاص الآخرة.
Habib ibn Abi Marzouq reported:
"Uthman ibn Affan, may Allah be pleased with him, entered his house and saw his servant playing in a camel's food bowl. Uthman took his ear and twisted it, but later he regretted what he did. Uthman ibn Affan said to his servant, "Take my ear and twist it." Uthman forced him to do it and told him to pull harder and harder, until he thought that what was done to himself was the same as what he did to his servant. Uthman then said: 'How easy is retribution in the world instead of retribution in the Hereafter!'"
[Tarikh al-Madinah 1018 from 'Umar ibn Shabbah, al-Ahwal #255 from Ibn Abi Dunyah]
Sa'eed ibn Jubayr said:
"As for the person who oppresses his son or brother, surely Allah will not let the one who does evil go unnoticed."
[Musnad Ibn Mubarak 178]
Consequence 3: Mental illnesses and trust issues
A child who gets beaten harshly will develop mental health and trust issues. This is a big consequence of hitting harshly, as it damages the child's self-worth and self-esteem.
The child will grow up not trusting anyone, which will lead him to miss out on many opportunities. He will either become violent himself or completely isolate himself from the outer world.
When those mental health issues grow heavier and worse, the person can become very depressed and eventually suicidal or display criminal activities.