LONDON — UNDER a comprehensive new law, British children will be seen and heard. And their rights will be paramount.Described by Prime Minister John Major as "the most far- reaching reform of child law this century," the Children Act went into effect Oct. 14 and brings together a wide range of measures aimed at protecting minors. It also sets up specialized family courts run by officials trained in family law. The system will depend not only on the definition of the rights of the child within the family, but on the effectiveness of local authorities in protecting children when the family bond fractures. The new law, the result of a seven-year government review of child-care legislation, introduces a new concept of parental responsibility. "It rests on the conviction that the state should not interfere with family life unless the child's welfare absolutely requires it," says Virginia Bottomley, Mr. Major's minister for children's affairs. "When intervention becomes necessary, the rights of the child must be elevated to the highest level." Where children are to be taken into care, local authorities must consult with parents and children about arrangements, and in some cases enter into strict agreements with parents, she added. Moves to introduce an "umbrella" law to safeguard children and give them inalienable rights were given a boost in the late 1980s by a succession of high-profile cases in which it became clear that children thought to be victims of physical or sexual abuse suffered either from parental cruelty or indifference, or from bureaucratic bungling. In a benchmark case two years ago, children in Cleveland, northern England, were removed from their homes by a local agency, which claimed they had been sexually abused by their parents. It later emerged that medical diagnoses on which the allegations were based had been flawed. The subsequent inquiry convinced Dame Elizabeth Butler-Sloss, who chaired the inquiry, that children's rights had to be protected by a comprehensive law backed by court administration. Dame Elizabeth is Britain's most senior female judge. "A child is a person, not a package," she said. "It can't just be picked up and deposited. Children are entitled to be consulted on what happens to them. The act is a marvelous step forward." Mrs. Bottomley says the law sets out new definitions of the rights and responsibilities of parents. Married parents will share legal responsibility for their children even if they later divorce. Unmarried mothers will have parental responsibility, but fathers will be able to gain it either by mutual consent or court order. Under the new law a child may request to live with either parent before a court decides. Even then the other parent will be encouraged to take an interest in the child and will have a legal right to do so. Bottomley said the new provision was aimed at easing the pressures on children in "tug-of-love" cases, although she conceded that some of the pressures would probably remain. Under the new law, when children are removed from home, local authorities will be able to hold them for only eight days, instead of 28 days as in the past. Parents have the right to challenge the removal after 72 hours. Britain's lord chancellor, Lord Mackay, said the greatest impact of the Children Act would be in "reinforcing the family as the natural unit for bringing up children." "It makes a deliberate attempt to ensure that children are looked after inside their families. It also creates adequate machinery to remove them from danger when they are at risk," he said. He stressed the importance of dealing with cases involving children expertly and without undue delay. "If the law is too slow, the children suffer."