HOUSTON—Prosecutors on Thursday charged a woman with capital murder in the death of a 5-year-old Houston boy she had been caring for and whose body was allegedly kept in a storage unit before being discovered in an East Texas motel. Theresa Balboa, 29, previously was arrested and charged with tampering with evidence, a human corpse, in the death of Samuel Olson, whose father Balboa was dating. Samuel’s cause of death was ruled a homicide, but the Harris County District Attorney’s Office had waited to charge Balboa with capital murder until investigating how he was killed. Court documents allege Balboa hit the boy “with a blunt object,” but did not provide additional details. Prosecutors previously said they had been testing a possible murder weapon. Robert Scott, Balboa’s attorney, did not immediately return a call or email seeking comment. Samuel had been living with Balboa since April 30, which was the last …