A Milwaukee fire killed six children as they slept in a crowded home where electric space heaters were being used because the gas had been shut off, bringing to 18 the number of people who have died in inner-city fires in three weeks, city authorities said. The mother of five of the six children, who ranged in age from 1 to 17, was away from home, hospitalized after giving birth to another child two days earlier, officials said. Neighbors who tried to rescue the children were driven back by heat and smoke. Firefighters found electric space heaters in the two-story, single-family home, but fire investigators said the cause had not been determined. A Sept. 30 fire that killed 12 people, including 10 children, occurred about a mile from the scene of the latest fire.