Sadly, the same engineering genius that can keep even ants out keeps oxygen out and carbon monoxide in. Tight gasket seal. Probably the threading that makes such a lockseal work renders it impossible to reverse the process from beneath, no match for a six year old, powered by panic and power-impaired by dark and diminished oxygen, but impossible for anyone, due to a design flaw, not unlike trunks of old which couldn't be opened from within, only because no one foresaw the critical need.
Not a submarine hatch which is designed to be locked and opened from either side, but in general, the barrel lid probably had a hatch similar in theory. By half. Perfect for food storage, fatal for egress.
Having recently lost my only child, my beloved daughter (at 28), my heart is broken for this mother. Such impossible loss. I share her pain.
JMO