This new series of observational documentaries explores a variety of rare and complex medical conditions and pioneering treatments at one of the most famous children’s hospitals in the world – London’s Great Ormond Street.

Filmed over eight months, the series follows 12 children and their families at home and in the hospital, as doctors diagnose and treat their conditions using cutting edge techniques.

This opening programme features Molly who suffers from a rare form of cancer, and Tahlia, whose windpipe is so narrow she can barely breathe.

Four-year-old Molly has a form of kidney cancer called Wilms’ tumour. The condition is normally found in younger children and is relatively easy to treat, but the cancer has gone unnoticed in Molly for so long that it has spread to her chest. Having already been through months of chemotherapy and had one kidney removed, Molly now has three life-threatening tumours which surgeons must remove.

But none of this seems to daunt Molly who remains happy and talkative at all times. “As ridiculous as it sounds,� says Rob, Molly’s dad, “you get by through the strength of your child.�

Molly’s consultant at Great Ormond Street is Gill Levitt, a specialist in Wilms’ tumour who has worked at the hospital for some 20 years. She explains that the tumours in Molly’s chest may still be active, but surgery to remove them would be life-threatening. Due to the proximity of one of the tumours to the heart, in fact, surgery may not even be possible. Levitt hopes that a new scanning technique – only recently being tested on children – will reveal if the tumours are active, without the need for invasive surgery.

Two days after Molly’s scan, it seems that Levitt’s technique has worked, but the news is not good. The cancer in Molly’s chest is still active, despite the chemotherapy, so she is booked into surgery with Professor Martin Elliott, one of the hospital’s top surgeons. Elliott and his team plan two operations to remove the tumours, but they are well aware of the risks involved. Because one of the tumours is so close to Molly’s heart, it may be too risky to remove it. The only way to find out for sure, however, is to go ahead with the operation. “It remains one of those rare bits of surgery in the modern era,� Elliott explains, “where you can only find out by doing it – which is not an ideal situation.�

As the surgeons prepare for the first of the operations, it is a very stressful time for Molly and her parents. Rob and Emma, Molly’s mum, try to remain strong for their daughter, knowing all the time that her life hangs in the balance. Dr Levitt remains positive, but admits that if the surgery fails, “we’re in serious trouble.�

Great Ormond Street does not have an accident and emergency department, so when a child arrives by ambulance, it can only be a very serious case. One-year-old Tahlia has been rushed to the hospital with breathing problems, and is taken immediately into the critical care unit while her parents look on.

It is only now that Alison and Paul discover that their daughter suffers from longsegment tracheal stenosis – a condition that means her windpipe is very narrow in places. The disorder, which can be present at birth or be acquired through trauma or infection, is so rare that only one in five million people suffer from it.

Many children in the past have died from Tahlia’s condition, but Professor Elliott and his team changed all that with a revolutionary new surgical technique that shortens and widens the trachea.

Tahlia’s situation, however, proves a challenge even for Elliott’s team. As she is rushed into theatre, the surgeons discover that Tahlia’s windpipe is less than a third of the size of that of a normal child. With a tube just 1.8 millimetres thick through which to breathe, Elliott reflects, “it’s incredible this child is alive.�

Sadly, after successful surgery to remove Molly’s tumours, the cancer returned and she died at the end of 2006. Her parents have asked for the programme to be shown as a tribute to her life.

This episode of Child In A Million will air on Channel Five on the 21st February from 8-9pm