"The fact that they had a test there available to her was paramount. It saved her life," said Stacey.
"It's a miracle she's here and we're extremely grateful for that," added Willow's dad, Russ.
With the right treatment, Willow was eventually able to recover from her infection, but needed several months in hospital.
She is one of many patients at the children's hospital to benefit from the new type of testing.
During a study in 2024-5 metagenomic testing was used on 122 critically ill children in the paediatric intensive care unit (ICU) at GOSH.
The results have now been published in the Lancet Microbe.
Metagenomic testing analyses all genetic material in a sample and can detect thousands of different pathogens.
By contrast, the most common method of identifying bacteria is by trying to culture them in Petri dishes, a technology that has been used since Victorian times.
Another test, PCR, can identify viruses and some bacteria, but doctors need to specify what they are trying to detect.
Metagenomics testing at GOSH was able to identify around nine in 10 bacteria and viruses and all fungi in samples.
In a quarter of cases, the results influenced prescribing decisions, including starting, stopping or changing an antibiotic.
Dr Sam Ray, consultant in paediatric intensive care medicine at GOSH and co-author of the study said traditional testing required clinicians to make an educated guess about which infection they were looking for.
"The study has helped us to spot infections we would have missed, get children on the right treatment and stop antibiotics when we know they aren't going to work.
"Removing unnecessary treatment is incredibly important for the fight against growing antimicrobial resistance."
Metagenomic testing at GOSH also detected 50 organisms not identified by routine diagnostics, meaning it could play an early warning role against new infections.
Dr Julianne Brown, head of the GOSH Metagenomics Service and co-senior author of the study said the new testing was being gradually rolled out to around 30 intensive care units across the UK treating both adults and children.
"It could be a powerful tool for identifying new infections that pose a pandemic threat," she added.
"In the future, if there was to be a new mystery pathogen that could be a cause of the next pandemic, we would detect that very quickly and put the right measures in place."
The project at GOSH builds on trials at Guy's and St Thomas' NHS Trust which first used metagenomics to test for respiratory infections in adult patients.