When a custodial parent passes away who gets custody of the child?

The surviving parent has the best claim to custody, in this situation.  Unless the surviving parent is unfit, or unable to care for the child properly, that's usually the end of the story.  There would ordinarily have to be some history of abuse or neglect, before the surviving parent's custody application could be contested with much likelihood of success.

After the surviving parent, grandparents, uncles and aunts, and adult siblings would be next in line.