OS ANGELES, March 11 - The sexual molesting case against Michael Jackson, which could put him in jail for 18 years, rests almost entirely on the confused and sometimes contradictory testimony of three siblings, none older than 18.

At the same time, defense lawyers for Mr. Jackson face a delicate balancing act in cross-examining the young witnesses. Challenging them too vigorously could turn the jury against the defense team and Mr. Jackson, who has promoted himself for years as loving children and all things childlike.

While the accuser and his siblings appear to be resolute on central aspects of the case, the halting manner of their presentation and the defense team's aggressive efforts to undermine them illustrate the difficulties in putting children on the witness stand and having their testimony stick.

Mr. Jackson's accuser, who was 13 when prosecutors say the abuse occurred two years ago at the singer's ranch, testified for an hour on Wednesday and all day Thursday. For the most part, he was composed and poised. But his memory failed him on several crucial points, and on others he contradicted his own grand jury testimony and the trial testimony of his younger brother and their sister, who testified earlier.

On Thursday, under friendly questioning from the chief prosecutor, Thomas W. Sneddon Jr., the accuser described an occasion when Mr. Jackson showed up naked and sexually aroused while the boy and his brother were on Mr. Jackson's bed. The accuser gave the grand jury that account last year, and his brother repeated it in court this week. The accuser told the grand jury that Mr. Jackson had told the boys his condition was natural and that he had discussed masturbation with them.

But on Thursday, the accuser testified that Mr. Jackson had not said anything, and the boy made no mention of the singer's aroused state. He then changed his testimony after Mr. Sneddon showed him what he had told the grand jury.

In another inconsistency, the accuser's 14-year-old brother said in court that he had twice witnessed Mr. Jackson sexually abusing the older boy, only to claim later that "there were actually three times."

Thomas Lyon, a law professor at the University of Southern California and an expert on child abuse, said that inconsistencies in testimony were common in molesting cases. He said that children in such cases can mesh repeated events together.

"The more they see something similar but somewhat different over time, it becomes blended in their memory," Professor Lyon said. A child, he said, "might be talking about lots of different events, and we don't know how much he's seen, and it's natural that it appears inconsistent in testimony."

Either way, Professor Lyon said, the defense's job "is to hammer on details any witness could have trouble with." Child witnesses, he said, tend to have more difficulty than adults in explaining inconsistencies.

The task for defense lawyers is made more complicated by human predisposition to sympathize with children.

"The defense has to walk a fine line between aggressively going after the inconsistencies and bullying the child," Professor Lyon said. But, he added, "once the child starts to crack a bit, the defense can be pretty aggressive."

Robert Weisberg, a professor of law and head of the Center for Criminal Justice at Stanford University, said defense lawyers can discredit child witnesses without grilling them. "It has to be done with kid gloves so it never appears to be attacking the witness," he said. Otherwise, he said, a cross examination could cause the jury "to take a dim view of the defendant."

Under cross-examination on Thursday by Thomas A. Mesereau Jr., Mr. Jackson's chief defense lawyer, the accuser's composure appeared to fade. He became argumentative and seemed unsure of his answers. Mr. Mesereau began to pick apart the narrative the boy had offered to Mr. Sneddon, and the boy showed uncertainty about the events that led up to the alleged molesting. The boy is to return to the stand on Monday.

Ronald Richards, a criminal defense lawyer from Beverly Hills who has been observing the trial, said the boy was "obviously very coached." But when the boy was pressed under questioning, Mr. Richards said, it exposed one of the dangers of putting a child, even a well-prepared teenager, on the stand.

"The witness was obviously eager to say what he'd been trained or coached to say in his extensive debriefings with prosecutors," Mr. Richards said. "The defense immediately grabbed the witness by the neck and brought him back to reality."

All three siblings denied they were trained for their court appearances. They were clearly embarrassed over the subject matter, and spelled out unsavory words rather than saying them. The three have also been reluctant to admit that their father beat them years ago; the younger brother told investigators at one point that the beatings had not occurred at all.

"That's a classic case of an abused child not wanting to disclose embarrassing information," Professor Lyon said. Such reluctance tends to be more pronounced when describing homosexual molesting, he said, because teenage boys are often highly aware of sexual identity issues.

The boy did not testify on Friday, when the session was dedicated to motions from lawyers. Prosecutors sought to obtain detailed financial information about Mr. Jackson, saying that he is in debt by as much as $400 million and "on the precipice of bankruptcy."

Prosecutors contend that Mr. Jackson saw his career and his financial empire crumbling as a result of a broadcast documentary in which he admitted sharing his bed with boys and that he took extreme measures, including a conspiracy to kidnap and imprison the family of the accuser, to contain the damage.

Judge Rodney S. Melville said he would limit evidence of Mr. Jackson's finances. "I don't think you can turn the jury into accountants," he said.

The judge also said that the comedian Jay Leno, who is expected to be a witness in the trial, can make jokes about Mr. Jackson if he avoids the facts of his testimony, which is expected to concern a call he received from the boy's mother.

Judge Melville refused a defense request to have jurors visit Mr. Jackson's Neverland ranch. The judge said the jury was seeing enough of the ranch through videotapes.

"I've never seen so many videotapes," he said, "and I'm sure I haven't seen the end of them."

Nick Madigan reported from Los Angeles for this article and John M. Broder from Santa Maria, Calif. Katie Zezima contributed reporting from Boston.