DAVID, our 15-year-old, inherited a nice little earner when his sister went off to university. He babysat regularly for several neighbours who had become family friends. One afternoon, one of them phoned. On the previous evening our son had looked after Jane, Anna's three-year-old daughter, while she and her husband, an American academic, went to the theatre.

Anna was hysterical. 'Can you come round right away? Something happened last night with David and it's real bad.' The accusation was devastating. Jane had been in bed, awake, flushed and over-excited, when her parents returned. Jane told her mother that David had 'opened' or 'touched' her bottom. The child spent a restive night. In the morning Anna had taken her daughter, still feverish, to a clinic. The specialist had found 'no evidence of anal or vaginal penetration'. But the specialist added that three-year-olds do not lie; an assertion that distressed me as it appeared to confirm the toddler's story.

It was all very civilised. There we sat, Anna and I, balancing coffee and cakes, discussing my son the child abuser. I found myself thanking her for taking the whole business so decently, while attempting to present a case for the defence. 'I know the boy looks like a Serbian irregular,' I heard myself saying. 'But he is a gentle soul and a vegetarian. You know as well as I do that he is not a loner or an introvert - he is a gregarious lad. He has been brought up among girls as well as boys, so 'bottoms' carry few surprises for him.'

I stumbled on. David had a longstanding girlfriend. I did not think they slept together, but actually had no idea. (Was my ignorance a sign of irresponsible and lax parenting? Would 'normal' sexual activity make child molesting more, or less, likely?) He did not babysit just for the money. He liked little children and spent a lot of time playing with our friends' youngsters. (Was I not pushing him ever deeper into the mire?)

Anna said that she and her husband did not yet intend to call the police. It would not be fair on Jane. But David had to confront them and admit his guilt, and we had to consider what treatment he should undergo. 'Suppose he interfered with some other child while babysitting?' she asked.

David was still at school when I returned home. I forced myself to phone that evening's clients and put them off with some unconvincing excuse about extra homework. Had I not done so, I feared Anna might have felt it was her duty to spread the accusation. In any case, if David was guilty it would have been wicked to allow him to continue babysitting. God alone knew how many other children he might molest.

Then a further refinement occurred to me. Perverse sexual behaviour is often learnt in childhood. Abusees became abusers. Who - if anybody - had abused David and so taught him to abuse others? I knew it was not me. But would the police and the social workers believe me?

To have a child accused of child abuse is a shattering experience. I simply did not know what to do. Luckily, a longstanding family friend is a child psychoanalyst. I phoned her before confronting my son. 'Nonsense,' she said briskly when I told her of the specialist's comment. 'Of course three-year-olds tell lies. The point is that they swing back and forth between fact and fiction without knowing what they are doing. At that age they are only beginning to learn the difference between fear and wish and reality.' A generation ago, she said, the tendency was to ignore a toddler talking of abuse. Now the balance had swung too far in the other direction. We were not to believe that an ambiguous accusation indicated guilt and not to be bounced or bullied by Jane's naturally distraught parents.

Finally, David came home from school. I confronted him. He sat, pale and catatonic, for some minutes. I felt reassured by his distress - until I realised that it could have been caused by guilt and fear as easily as by innocent incomprehension and outrage. Eventually he burst out: 'Why me? They're supposed to be our friends. Why are they telling lies? I love Jane. It's not fair.'

I said that I would believe whatever he told me, but had to ask harsh questions. Other people, starting with Jane's parents, certainly would want to know what had happened. 'Why are you doing this to me?' he replied. 'Whose side are you on? If you really don't believe them, why ask me anything? And why have you cancelled tonight's job? Just tell them to leave me alone. They're liars.'

I started the interrogation. 'I've got to ask you directly. Did you do anything wrong?' 'No, Dad.' 'You know, if anything did happen, you must say so for your own sake. However awful it seems, things will be worse if you hide it and let it drift on. If there is something wrong, we must sort it out now. Whatever has happened, you know your mother and I are on your side. We can get you help if you tell the truth. Now . . . Did you do anything wrong?' 'No Dad, no. For God's sake listen to me.'

By this time David was crying, but I continued to push. What had happened last night? He had, he claimed, put the girl in bed as usual but she was restive. She used the lavatory. Afterwards, he wiped her bottom and examined it to make sure that it was clean, as he had been taught to do. Then he put her back to bed. He had tickled her when she refused to settle down. He knew it was stupid to have done so - because it wound her up instead of calming her - but she had wanted to play.

In desperation, I promised that if he told me the truth, I would suppress it and if necessary lie to protect him. How I would have reacted to a confession, I do not know. The issue did not arise. David still protested his innocence.

The hardest part was to try to explain to a 15-year-old with simplistic notions of right and wrong that life is not fair, and he had better get used to it. Talking in such brutal terms seemed a violation of his innocence and I resented being compelled to do so. I told him that life inevitably includes injustices - some favourable and some not.

Two days later the families (minus Jane) had what Anna described as a conference. She reported that Jane was calm and happy and that her doctor had diagnosed nothing more dramatic than a feverish cold. David stumbled through his account of the unhappy evening, Anna and her husband accepted that nothing improper had happened. Self-consciously, we all shook hands.

Three months on, David has just felt able to start babysitting again, though not with Jane. She smiles and waves when we meet her in the park. We adults cannot bring ourselves to acknowledge each other.

Names have been changed.