Michael Jackson is dead and has been for almost 13 years, and therefore whatever he did in his personal life is no longer something we will ever know, nor is it of any real import at this point.

Michael Jackson has always claimed his three children, Prince, Paris, and Blanket (don’t remember his real name now) as his own. He raised them, provided for them, and publicly declared them to be his kin.
The Jackson family recognizes the children as his as well and accepts them as a part of their family. Michael’s mother, Kathryn Jackson, took the children in after his death and willingly acted as their legal guardian as far as we know.

The children themselves recognize Michael as their only father and apparently had real affection for him.

At this point, who cares if they look like Michael or not? At best, if they are not his biological children, they are certainly his adopted children. And adopted or not, he is/was still their father. Whether or not he is their biological father just shouldn’t matter anymore. They are all adults now and they are doing their own thing.