RIVERSIDE, Calif. (AP) _ Sixth-graders vowed Monday not to ''give up giving'' after learning that $110 they sent for an 11-year-old cancer victim instead went to a woman who invented the boy as a way to trade postage stamps.

When classes resumed Monday after Christmas vacation at Grant Elementary School, reading teacher Pat Dahlberg opened a registered letter that contained the returned check and a letter signed by the fictitious Tommy Martin in New York City.

''Mom said we can't accept any money. I hope the kids won't be mad at me,'' said the letter.

The children's involvement in the hoax began when Kay Stamps, a friend of Ms. Dahlberg's, placed an ad in a stamp collecters' magazine seeking pen pals who would want to trade stamps.

Ms. Stamps received a reply from the purported 11-year-old and sent $20, cookies and books, then mentioned the situation to Ms. Dahlberg, who launched her class on the charitable project.

Principal Patsy Martin said the 36 youngsters, who had held a bake sale to raise the $110, planned to use the money for some other worthy cause - perhaps to aid Riverside Community Hospital or to help starving children in Ethiopia.

''The children are wonderful.... They reacted that they are a little more wary about who they donate to, but they came up with a slogan - 'Don't give up giving,''' she said.

Ms. Martin said the children at first were ''horrified'' there was no Tommy Martin, then were angry at the 59-year-old woman who had fabricated ''the poor little boy who had lost his hair through chemotherapy.''

''They were also concerned about others, because... certainly there will be others writing to her,'' the principal said.

The letters the children had received from ''Tommy'' said the boy's father had been robbed and murdered while driving a cab, that his mother was on welfare and that the hospitalized boy couldn't watch television because he couldn't get into his wheelchair to go to the hospital television room.

''The kids were so upset when they heard that,'' Ms. Dahlberg said. ''They all said, 'we've got to send him a TV.'''

The principal said the children ''would have been happy to write her letters, send her stamps if she had just been honest with them.''

''The big problem is they know people who really are dying of cancer, so their hearts went out to this cild who is their age,'' she said.

''There is no Tommy Martin. There isn't one, period - dying of cancer or healthy,'' said New York attorney Paul Chazan, who knows the woman through his involvement in a volunteer organization that visits elderly and poor people.

Chazan said the woman wanted nothing more than stamps. He said she assumed the role of a child in her letters, because her handwriting was so poor that nobody would have believed she was an adult. He didn't know why she invented the cancer story.

He said she works at a factory, earning $5 an hour, and shares a small apartment with another woman.

''They are lonely. They have so little,'' he said.