The coroner in 1986 said Kylie died from being strangled, but i have to wonder whether she was already fatally hurt from being given an overdose of Valium. Remember, Davies wasn't a doctor and would have no idea of the safe dosage to give a small child - he just wanted her too drugged to scream.
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Usual Pediatric Dose for Light Sedation
Conscious sedation for procedures:
Oral:
1 to 12 years: 0.2 to 0.3 mg/kg orally 45 to 60 minutes before procedure, up to a maximum of 10 mg
13 to 18 years: 5 mg orally 45 to 60 minutes before procedure, may repeat with 2.5 mg dose.
Sedation:
1 to 12 years:
Oral: 0.02 to 0.3 mg/kg every 6 to 8 hours as needed.
IM: 0.04 to 0.3 mg/kg IM every 2 to 4 hours as needed, up to a maximum of 0.6 mg/kg in 8 hours.
13 to 18 years:
Oral: 2 to 10 mg 2 to 4 times a day as needed.
IM or IV: 2 to 10 mg 2 to 4 times a day as needed.'
https://www.drugs.com/dosage/diazepam.html
You can download a weight for age chart here and do the calculations:
http://www.education.vic.gov.au/Documents/childhood/parents/mch/mchgrowthbmigirl18yrs.pdf
The weight range for a 6 year old girl is between 15-27 kgs.
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Usual Adult Dose for Anxiety
Oral: 2 to 10 mg 2 to 4 times a day.
IM or IV: 2 to 5 mg (moderate anxiety) or 5 to 10 mg (severe anxiety) for one dose. May repeat in 3 to 4 hours, if necessary.'
https://www.drugs.com/dosage/diazepam.html
If Kylie was given an adult dose of, say 10mg, I'd say that would be enough to sedate her but I am not a medical professional.
Is there a Coroner's Report with toxicology results available?