CAMBRIDGE, Mass. (AP) _ The dialogue sounded like that of any party: announcements of guests arriving and departing, jokes about the cake, even a chorus of ''Happy Birthday.''

But this was no ordinary party. The guests were in different cities, the only sound was the crisp clicking of key strokes and the birthday boy was missing, taken from a father who faced a mournful day remembering his loss.

The gathering took place on home computer screens around the country, organized by members of Child Net, a computer network that offers help and hope to parents of missing children.

''It was tremendous support,'' said Richard Charm, a Hamden, Conn., locksmith who celebrated his son Michael's seventh birthday in absentia with a half dozen other people via personal computers.

''I thought I was the only person on the face of the Earth who had this problem,'' said Charm, whose son disappeared more than four years ago. ''With the computer, I got a lot of help from other parents who have missing children or recovered missing children.''

Child Net was started last year by David Landrigan, a University of Lowell psychology professor and computer user, after his 4-year-old daughter, Angela, was taken by her mother.

After frustrating weeks of contacting individual groups specializing in recovering missing children, Landrigan turned to his computer contacts.

''When Angela was missing, the law enforcement agencies were helpful, friends were helpful and organizations were helpful, but no one could do that much,'' he said.

From his Cambridge home, however, Landrigan was able to generate pictures of Angela over computer networks, allowing people around the country to print out posters and place them in public places. A salesman who had seen one of the posters spotted a child resembling Angela in Texas and alerted authorities.

Three months after she disappeared, she was recovered outside Austin.

From his success, Child Net was formed.

''It came about as a result of seeing the need for people to work cooperatively on these cases,'' Landrigan said. ''It was not just a matter of getting the picture out there, it was a question of people sharing information.''

Today, Child Net offers more than 1,700 ''conferences,'' ranging from reports on individual missing children to group discussions on child safety, legal issues, news and information. Everything relates to children.

Anyone with a computer and a modem, a device that allows computers to communicate over telephone lines, can get on Child Net by calling into Networking and World Information Inc., an computer-based information exchange in East Hartford, Conn.

By typing simple commands and answering questions that appear on their computer screens, people can join different ''conferences'' within Child Net, read what others have written and leave notices, information and words of support for other parents.

Participants commonly find messages waiting for them when they sit down at their screens and enter Child Net, perhaps a note about a development in a case they've been following or additional debate on the general question of parental custody. They may add opinions, suggestions or support, simply by typing a message and sending it to a specific conference.

Human drama permeates every flickering screenful of Child Net conferencing. On line you can meet Deputy Duck, an Indiana deputy sheriff who offers tips on child safety. In another conference an Ohio father, stymied in efforts to reclaim a son taken to England by his ex-wife, is given a contact number in the State Department that clears a legal path through the thicket of international red tape.

Under a conference named ''Cathy,'' for Catherine Malcolmson, a 16-year-old Stow, Mass., girl who vanished in 1985, an on-line friend offered this birthday wish to her parents: ''Cathy wherever you are ... you have so many people thinking of you on this your eighteenth birthday ... if you are with us on Earth, I hope that you are happy ... if you are not with us, I KNOW that you are happy in heaven.''

Some 600 to 700 missing children are listed in Child Net, which has plans to develop an on-line, computerized registration system that would allow parents to fill out one form that would then be shared with the 100 or more groups involved in the search for missing children.

In any case involving a dispute between parents, the organizations that feed information to Child Net require a copy of court-issued custody orders to assure the parent seeking the child has a legitimate claim.

Landrigan estimates that more than 1,000 children registered with Child Net member organizations are recovered each year, often through information provided through the network.

''There were lots of little organizations in places like Newport, R.I., Los Gatos, Calif., and Largo, Fla., unable to reach out to each other or get in touch with volunteers,'' said Landrigan. ''Now they can.''

Peggy Berk, a spokeswoman for Networking and World Information Inc., said it's hard to gauge how many people receive information from Child Net. She said that while about 1,000 have gone on line with the service, the information from Child Net can be transferred to other computer bulletin boards, giving it an audience in the tens of thousands.

''We get responses from all over the world,'' she said. ''It has become very much a community.'' One community member is Ken Jones, a San Jose, Calif., landscape consultant who learned of Child Net last year when his 7-year-old daughter, Cori, disappeared with his ex-wife.

Jones, who used a home computer for his work, was able to transmit pictures of his daughter to computer bulletin boards via Child Net. He also received invaluable support from other parents through the computer line.

''I probably talked to 30 to 40 people on line,'' said Jones. ''I was on line virtually every night, talking live with people.''

Cori was found in Southern California earlier this year after a school employee noticed her picture on a poster distributed by the state school system. Although Child Net was not instrumental in recovering his daughter, Jones remains a member of the group.

''I have become close friends with a dozen people whose children are missing or who had been missing,'' he said.

While Landrigan acknowledges Child Net's usefulness in providing expertise and information to frantic parents, he also emphasizes its power to soothe, by permitting computer conversations between people thousands of miles and several time zones away.

''If a parent is having a bad night, they can get up at 3 in the morning and go on line,'' he said. ''Most people wouldn't think of calling a close friend at 3 a.m. This way you can always find someone to talk to.''

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Eds: Fred Bayles, an AP national writer, went ''on-line'' with a personal computer for this story. Those interested in Child Net can get more information by calling Networking and World Information at 1-800-624-5916. In Connecticut the number is 1-800-624-5958.