When I was 5 I first thought about death. I was raised Catholic and had been taught that good people went to heaven when they died, and lived there in a state of great happiness with God, forever and ever and ever and ever... Such existence seemed terrifyingly boring to me. (And when I write "terrifyingly" I mean exactly that.) Then again, the alternatives of hell and purgatory seemed just as boring, but also painful to boot. I concluded that "life after death" was horrible no matter how I sliced it. And therefore death was terrifying. Thinking about these things made me very scared, but I trusted my elders, and I hoped that when I grew up I would understand things better, and would see that there was nothing to be afraid of...
I think this counts as a childhood belief because I thought it all out on my own as a kid (I even remember where I was when I did), but it was pretty much the way I viewed the matter for as long as I remained Catholic in my mid teens. I have been an atheist for a long time now. I find death as scary as I did when I was five.
A childhood belief about afterlife

others believed things like this
- I used to think that if you were scared "half to death" that you only had one scare left before you died.— Bush Hiddenofsky
- I used to believe that "Earwigs lived in your ears", scared
me to death for years.— Will Chapman - I thought that when I got older "they" would invent a cure for death and I could live forever.— wishful thinking
- When I was little, I used to believe that my Barbie's would come to life while I was asleep to plot my death!— Kimbre
- I used to be afraid of moving my wrists incase i snapped my veins and bled to death.— Tender veins
- I believed one of the greatest dangers you could face was quick sand. I was scared to death of it.Anon