The desert, without water
Outer space, running out of air
Poison to the touch
The abominable snow man
Invaders from Mars
These were the things I worried about as a kid. I thought that adults had to deal with this stuff all the time. That's what you get for watching SuperFriends and Speed Racer and Land of the Lost and Lost in Space.
About the last item -- seven-year-old American boys are prepared for sudden violence against the British anytime, anywhere. Sorry, Brits, don't even think about taking back your colonies.
About being stranded in the desert with no water -- you always hope for an oasis, but you're scared it might be a mirage.
Deep in the African interior is an ancient hidden lake, on the shores of which homo sapiens is rumored to have evolved. Travellers who have returned from there claim that wading into the water feels just like the moments before falling asleep; its buoyancy and warmth and motion are encoded in our genetic memory.
Some call it Lake Eden, others call it the lake of dreams.
Its location has been reported as far north as Chad and as far south as Botswana. Recent U.N.-sponsored expeditions have returned without success.