Janet Kiesner's 5-year-old son must walk to the same school his older brother goes to by bus because of a district policy designed to keep down costs. ``It's so dumb. I'd never have enrolled him (in kindergarten) . .. if I had known this would happen,'' said Kiesner, who doesn't drive. ``But now he loves it and I don't want to pull him out.'' Her son Joey is not permitted to take the bus because the Milwaukee Public Schools System prohibits the busing of children in its kindergarten classes for 4-year-olds if the youngsters live less than a mile from school. Older pupils, including those in kindergarten for 5-year-olds, get bus service. The problem, according to Richard F. Wenzel, assistant director of transportation for the district, is state law. It requires that if bus transportation were provided for pupils in 4-year-old kindergarten, it would have to be offered to all children in that age group. ``That would include 4-year-olds at day-care centers all over the city as well as to five miles outside the city,'' Wenzel said. ``There would be quite a financial impact.'' ``If it's too hazardous for a 5-year-old or my third-grader to walk, certainly it's too dangerous for a 4-year-old,'' said a frustrated Kiesner, whose 8-year-old son gets bused to the same school. She said she walks a neighbor's daughter, a 5-year-old kindergarten pupil, to her bus stop each day, waits for the bus to pick up the girl and then walks her younger son about six blocks along a busy city street to get him to the same school. ``To put one little kid on that bus and then have to walk my own child up to the same school seems ridiculous,'' Kiesner said. ``It's not like I'm asking for a special bus. It already stops right here and there's plenty of room,'' she said. School officials said they would like to bus Joey, but they are restricted by the school board's policy. And even though Joey has turned 5, he is still covered by the policy because he remains enrolled in 4-year-old kindergarten. The board has directed Superintendent Robert S. Peterkin to review the district's student assignment process, but Peterkin said no changes are likely to be made until 1990-91. School officials said there are two options for Kiesner. She can select another school farther from her home. According to Wenzel, busing is provided to youngsters in 4-year-old kindergarten if they attend a school more than a mile from home and their attendance enhances the school's racial balance. But Kiesner said she wants to keep her two children in the same school. ``I feel better knowing he (Joey) is in the same school with his brother.'' The second option, Wenzel said, is for Kiesner to negotiate a private contract with the bus company. A bus company spokesman estimated such a contract would cost about $2.50 a day. ``That's beyond my budget. I couldn't afford that,'' Kiesner said.