Muskegon Police Chief Jeffrey Lewis said the toddler's body was found concealed Friday morning in a bedroom. He said the primary witness is only 5 years old.
Lewis said police did not yet know a cause of death.
The 1-year-old had been crying and "the 8-year-old dealt with the baby crying,” Lewis said.
Bryanna Reasonover told WZZM-TV that she found her son's body in a playpen when she went to pick up three children she had dropped off the previous night.
The 8-year-old was taken to a hospital for a mental evaluation.