The child does not come from all the liquid (sermen) and when Allah intends to create anything nothing can prevent it (from coming into existence).  

عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ سَمِعَهُ يَقُولُ سُئِلَ رَسُولُ اللَّهِ ﷺ عَنِ الْعَزْلِ فَقَالَ مَا مِنْ كُلِّ الْمَاءِ يَكُونُ الْوَلَدُ وَإِذَا أَرَادَ اللَّهُ خَلْقَ شَىْءٍ لَمْ يَمْنَعْهُ شَىْءٌ  

[Machine] The Messenger of Allah ﷺ was asked about al-'azl (withdrawing before ejaculation) and he said, "Not every fluid becomes a child when Allah desires to create something. Nothing prevents Him."  

سُئِلَ رَسُولُ اللهِ ﷺ عَنِ الْعَزْلِ فَقَالَ لَيْسَ مِنْ كُلِّ الْمَاءِ يَكُونُ الْوَلَدُ إِذَا أَرَادَ اللهُ أَنْ يَخْلُقَ شَيْئًا لَمْ يَمْنَعْهُ شَيْءٌ  

"إذَا أراد الله خلقَ شيءٍ لم يمنعه شيءٌ".