"No, no, no," Joseph Dillon scolds the personal computer in frustration. But with a few deft moves of the mouse, he solves the problem. The image he wants pops up on the screen.

"E-e-e-e-e," he squeals with joy, smiling and clapping his hands.His anguish and elation are shared by millions each day as they confront computer technology, even if most are not so demonstrative.

But then, 2-year-olds don't hide their emotions. What so delighted Joseph Dillon was the image of a clown with balloons.

The next wave of keyboard conscripts is toddling into the digital age. They may not say much or be toilet-trained yet, but they are a lucrative new market for software producers.

Sales of software for children ages 5 and younger more than doubled last year, to $41 million, according to PC Data Inc., a research company in Reston, Va., that tracks the industry. For its calculation, PC Data counted software programs sold in four sets of recommended age groups - 18 months to 3 years, 2 to 4, 2 to 5 and 3 to 5.

Introducing children as young as this to computing, however, is a subject of heated debate among educators and child-development ex-perts.

"Computers are transforming our society in both good ways and silly ways," said Judah Schwartz, co-director of the Educational Technology Center at Harvard University. "And this seems to be one of the sillier ways."

Nothing silly about it, some child-development specialists reply. Personal computers, they insist, will soon be as natural a fixture in the playroom or nursery as Dr. Seuss or Babar.

"Just as books are adapted in both form and content to meet the needs of babies and toddlers, computers and software can be adapted to delight and educate even the very young," said Corinne Rupert, a child psychologist. If the software is properly made and easy to use, she declared, "There is no minimum age level to computer introduction."

Indeed, Rupert served as an adviser on Jumpstart Baby, a software program that is scheduled to go on sale next month and that epitomizes the youth movement in computer software - it is recommended for children from 2 years down to 9 months old.

Not everybody agrees that children should be taking time away from blocks and dolls to sit in front of a computer. Such talk has even longtime analysts of the computer industry shaking their heads in amazement.

"What's next?" asked Ann Stephens, president of PC Data. "Wombware?"

Though difficult to measure, forces encouraging sales include social pressure and parental guilt. At every turn, it seems, parents are told that if their children are not trained to join the information revolution, they will be condemned to lives of benighted poverty.

In the future, the argument runs, children will be facing increasing competition to earn good grades and get into good schools. So parents, especially the well-educated progeny of the baby-boom era, are trying to give their children any edge they can, no matter how early in life.

"Boomer parents are taking every step they can to make sure their kids are prepared for Harvard right out of the womb," said Robin Raskin, editor in chief of Family PC magazine, a monthly. "And the preschool and toddler software is selling to those parents."

The software industry promotes its early-age titles as tools for developing "prereading skills," "mouse literacy" and "respect for the computer." Judging from recent sales trends, the marketing message has proved effective. Sales of software for children 5 and under rose from 179,000 units in 1995 to 1.5 million in 1997. The real surge, notes Stephens, came in the last few months of 1997.

As recently as four years ago, children's software was dominated by titles for older children like Mavis Beacon Teaches Typing (recommended for users 12 and older) and Where in the World Is Carmen Sandiego? (9 to 14).

Today, the preschool offerings are joining the best-seller list, including winners like Sesame Street Elmo's Preschool (2 to 4), Ready to Read With Pooh (3 to 6), Jump-start Preschool (2 to 4) and Jump-start Toddlers (11/2 to 3).

The trend is disturbing to Clifford Nass, a professor at Stanford University, who specializes in the interaction between people and com-puters. Young children, he says, learn best when they are playing with real objects, like puzzles and teddy bears, along with other children or adults. The social context and the tactile experience are crucial to early development.

"I think it's a really bad thing for kids to slap them in front of a computer screen at a very young age," Nass said.

His worries are valid but overstated, software experts and executives say. Toddler software, like anything else, must be used properly to be of value, they say.

"Computers will never and should never replace story books, crayons, blocks or Play-Doh," said Warren Buckleitner, editor of the Children's Software Revue, a newsletter for parents.