No clue how it works in the UK, but here in the States, any unexpected death (which is the vast majority of pediatric deaths) is automatically an ME case. Now, this does not mean it is done well, but there is at least the appearance of an investigation. But, as I said, not familiar with UK SOP.
Apparently although they were reporting the deaths, they were either not reporting correctly or the people they reported to were not actually looking at the data.
A spike in deaths like that should have had red flags popping up all over.