I realise I spoil the children in one home more than the other. How do I stop?

Admitting that tendency is the first step; next, correct it deliberately and consistently. The Prophet ﷺ said: “Be conscious of Allah and act justly among your children” (Sahih al-Bukhari 2587). Start by reviewing the things that can be measured — time, gifts, praise and support — and close the gaps you see. Fairness does not mean being exactly equal in everything, but every child has the right to feel equally loved and equally important. Be careful of jokes or comparisons that hurt one of them. If a child has already been affected, admit your mistake to him honestly; that teaches him that your love is undivided. Further guidance is in children in polygyny.