A Chicago boy whose family was preparing to celebrate his fourth birthday has been killed in what police say was a "possibly domestic situation".

Christopher Valdez was found dead in his home on Friday, shortly before his birthday party was supposed to begin, the Chicago Sun-Times reports.

The little boy was dead when police arrived at the home and appeared to have suffered from 'multiple blunt force trauma’.

Though police have not given away more details, it is understood that Christopher’s mum Crystal Valdez phoned one of her brothers on Friday, "desperate for help".

One of the brothers reportedly arrived on the scene, restrained Crystal’s boyfriend and discovered that the young boy had been beaten.

"My niece's boyfriend beat the baby to death, and I don't know exactly what happened. I've never even met the man," said Steven Valdez, the boy's great uncle.

The boyfriend has been taken into custody and the family is in shock. Relatives say they ahd gathered for Christopher's birthday with cake and presents but are now in mourning.

"I just can't understand how anybody could do that," said Santiago Valdez, the boy's great-grandfather. "I just can't. It's just a shock to me."