the death rate aged 0-18 was almost TEN times its present level...Due to what?

I don't know. The figures were UK government figures, somewhere in the national statistics office. I'd guess at:

- a huge decrease in perinatal and neonatal mortality
- More vaccination and against more diseases (polio/ measles etc.)
- decreased road accident rate
- better housing and less air pollution
- Fewer young people working and work conditions safer
- the rest down to lowered accident rates in the home/ play etc.

But that's only using common sense :)