The parent of a child in a Tulare County school is threatening to keep her child home next week because a 5-year-old girl with AIDS is attending classes. But the fears of most other parents appeared to be calmed after a meeting last week attended by officials of the Roche Avenue Elementary School in Porterville and the Tulare County Health Department. Between 100 and 150 parents and staff members attended the meeting to hear a discussion about the facts surrounding AIDS. The child is believed to be the first AIDS victim attending Tulare County schools. The child's condition was known in her neighborhood and church before school enrollment, said Jacob Rankin, Porterville School District superintendent.