William had bone graft surgery last week. He's 11, amazing, and my son. Although we expected him to have this operation as part of treatment for cleft lip and palate, it was very last minute - a phone call from hospital on Tuesday afternoon to come in for the operation at 7:30am on Thursday. A cancellation meant they had to fill the surgical list quickly.

On the plus side, it meant my son had no time to get anxious. Friends and my parents rallied round to step in and help marshal our other three children. We drew up a schedule for the Thursday - 25 points and counting - so that in our absence, the grown-ups at home had some semblance of knowing what on earth was going on. I forgot to put 'listen to Elizabeth read for her homework' towards the end of the list. I fretted about it, because it was easier to focus on minor oversights than think about what William would have to undergo the next day.

Which is what, exactly? Cleft lip and palate is a treatable condition. It's not life limiting, in isolation. But it can cause, among other things, hearing problems, speech challenges, psychological issues because of looking and feeling different, long and difficult dental and orthodontic work. He's experienced all of these. Cleft lip and palate also needs surgery, right up into adulthood. This was William's seventh operation. He'll have more when he has jaw surgery as a teenager. He'll have his jaw broken and realigned, with years of braces, bridges and implants too. Many children without cleft can have glue ear, or speech therapy, or braces. For William and many other children with cleft, it's all of them combined. All of his treatment, ranging from audiology, speech therapy or seeing a psychologist, has been on the NHS.

As the oldest child, he was last on the list: nil by mouth from 6am until he went into the operating theatre at 4.30pm that day. All borne without a complaint and pretty much in silence the whole day. The two babies who were before him were both in theatre for three to four hours, having their cleft lip and cleft palate repaired. William kept busy watching films. We didn't get impatient. There's no point.

William's operation involved taking bone from his hip and inserting it, and bone marrow, into the gap in his gum caused by the cleft lip and palate that were both repaired when he was a baby. We've everything crossed that this time the bone graft will work: the first one, in 2012, didn't.

When you go down to the anaesthetic room with your child, you're handing the most precious thing you have to strangers - albeit ones whose entire professional lives are dedicated to making sure they don't mess up. The consequences don't bear thinking about. When your child comes out of surgery the recovery team is there to make sure your child wakes up safely, so they hear kind words and feel gentle hands as they change your child's gown after they've vomited blood into a cardboard basin after the accumulation of blood in their stomach after surgery. When you go down to collect your child with the ward nurse, who grabs a 'Finding Nemo' swimming bag of resuscitation equipment each time she goes down in the lift, just in case your child needs it on the journey from recovery to ward, you dimly realise she's gone past her 12-hour shift, but she's there anyway to help your child get back safely to the ward before handing over to the nurse on night duty.

It's not a question of ceding control which makes the experience of seeing your child, in the most vulnerable, potentially dangerous medical situation, being look after by other adults, so emotional. Rather, it's an appreciation of the NHS professionals who looked after my son at the end of last week - a conservative estimate of a dozen people - who spend their lives caring, clearing up, communicating with each other, measuring, checking, soothing, joking with the children, smiling, giving parents a reassuring hug, calmly going about their job. You can say 'thank you' again and again, but in truth you will never really be able to adequately express your gratitude to them all that when it came to your child, they didn't mess up.

Most importantly, this is an appreciation of my son. On the morning of surgery, a teacher popped by his bed to urge him to come to hospital school. There were about 10 kids at school that morning. He was the only pupil in Year 6, and spent the morning with bandages pre-cannula on his hands doggedly learning the more obscure capitals of South America and Asia, doing PE with small kids in Years 1 and 2, and finally having an Art lesson before lunch (which he obviously couldn't eat). He drew and painted this freehand.

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To me, this painting exemplifies William. The fact that he was able to concentrate and create this beautiful picture from memory, under a massive amount of pressure and nervous anticipation, knowing he was going to have to undergo a difficult operation later that day, says so many positive things about him. The older he gets, the more he understands what's involved. Like I said - he's amazing.