Madison, WI – Carbon monoxide from an old portable propane heater killed two children and their grandparents as they slept in a combination camper-horse trailer ahead of a horse show, authorities said yesterday. The four were discovered by relatives who went to wake them about 7:30 a.m., just as the World Clydesdale show was starting at the Alliant Energy Center. Dane County Coroner John Stanley identified the victims as DeVere Clay, 68; his wife, Barbara, 57; and grandchildren Erin Briney, 10, and Hope Briney, 13.