Yes, Islam commands justice between children, and favoring one over another in gifts is not permitted. When al-Nu'man ibn Bashir's father gave him a gift and asked the Prophet (peace be upon him) to witness it, the Prophet asked whether he had given the same to all his children; when he said no, the Prophet said, "Fear Allah and be just between your children," and refused to bear witness to it, calling it injustice. This applies to material gifts; as for matters like extra care for a sick or younger child, scholars note that need-based attention is not the injustice being forbidden.
Evidence — the daleel
Sahih al-Bukhari 2587 ṢaḥīḥSahih Muslim 1623 Ṣaḥīḥ
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.