Sean ruined primary-school Christmases telling everyone the truth about Santa. A boy called Peter ruined ours. Like most people I lied to my children about the man in a red suit who brings gifts. I also claimed that fairies need teeth.

Once our children find us out on both these counts, there’s often an unwillingness to discuss it. We put this down to the younger generation’s mercenary calculation that if they let on, the presents will dry up. More honestly, the reason neither side raises the subject of Father Christmas is that parents know they have been a disappointment and children know it too. The more investment both sides have made in the lie, the more difficult it is articulating why, even when it comes to Santa, a lie can feel like betrayal. Rather than talk about it many of us will play out the pretence for years with the stocking and its tangerine, because all of us wish that the magic were true.

The reason children are more gullible with their parents is because they want to believe those whom they love. They make assumptions. The least they expect is that their parents will tell them the truth. When we lie to them, they are put in the difficult position of having to choose between trusting themselves and trusting us.

To trust is also simpler. The lying landscape is confused. Children are told that lying is a crime. Sometimes they are beaten for it. Yet within hours and days they will hear a parent doing it, or even in some cases find themselves being asked to lie on that parent’s behalf.

We coax those in our care to keep secrets – the visit with a part-time parent where Roger fell into the pond, or Edith was taken to A&E, a pea lodged up her nose, force a child to collude with one adult, while alienating them from the other. These lies are provoked by self-interest. It is a behaviour children recognise from the playground, where truth privileges the few.

The lies that parents tell can be coercive. Some of us, facing another meal without any eaten vegetables, have resorted to the word ‘police’. Psychologists, when speaking of these lies, use the term ‘instrumental’ as if there were something innocent in telling a child that you will kick them out at the next service station, or if they don’t quit sucking their thumb the nasty Struwwelpeter man with big scissors will come chop it off.

Parenting is a not a job that comes naturally. Often we take the easiest option, following blindly the example of those who have gone before. If deceit has been a tool used against us, under pressure we tend to fall into bad habits and use that self-same tool ourselves. There’s nothing like facing the gummy innocence of a primary-schooler asking: ‘How come Jane’s daddy was crying at school?’ or ‘Why did Auntie Barb fall over?’ to force those of us who should know better into making it up on the spot. The whole truth – like Jane’s mummy is having an affair with that smarmy father in year five, or falling over is what happens when Auntie Barb has had too much to drink – just doesn’t seem appropriate. Why wreck innocence? It is this whole truth many argue that we are guarding against, because the truth is more information than anyone under sixteen needs.

Infidelity is one of those crimes that come under the whole truth umbrella. Fifty-seven per cent of men and 54 per cent of women admit to committing infidelity at some point in their lives. I have done it. Perhaps you have done it. It is a betrayal between two adults, and is unseemly detail that children, we tell ourselves, would not understand. Nor is it anything, we say, that affects them directly. Infidelity is just a symptom of more substantial problems, and under these circumstances we tend to withhold.

Withholding is a parental favourite. However, like a great novel, where plot is lovingly revealed through behaviour, silence is often partial. The clues are all around: finding one adult on the sofa every morning, and the other furtively going through the texts on the other’s phone, tells a story that seethes with questions which children are too afraid to ask.

Silence often makes a subject impossible to broach. It is lying without us having to face the fact that we have done so and is a form of deceit sanctioned by the Catholic Church. The doctrine of Mental Reservation stems from St Augustine’s belief that hiding the truth does not qualify as deception. Mental Reservation is a way to fulfil an obligation both to be ‘truthful’ and to keep a secret that is Catholicism at its worst. For instance, although senior clergy in Ireland said they had co-operated with police who were investigating allegations of crimes against children, they omitted the word ‘fully’.

In the case of infidelity the betrayed parent may choose to ‘protect’ children from the truth because they want to avoid being the bringer of bad news. Better to sacrifice totally to betrayal than risk being labelled the baddie. Poor Mum.

Though the Protestant God she prayed to did not have the flexibility of his rival, who allows absolution to be dished out by the priest, she shared a bed with a man who was well versed in the Augustinian principles. Dad understood that as long as a Catholic can stay alive for the time it takes a priest to administer the Last Rites, heaven awaits. In fact he was so motivated to have this final forgiveness that on Monday evenings he trained up my mother’s Episcopalian minister to perform it when the time came. The Last Rites are like declaring yourself bankrupt without losing the house or the car.

But perhaps I am looking at all this the wrong way round. In the end Dad could not trust his children to allow him to be his true self. Particularly me. He colluded, hoping to protect me from my own disappointment. He understood that what I wanted, like the man in a red suit with presents, was the father that he was not.