Why are so many kids' movies so scary?

My kids were terrified by the scene in Finding Nemo where Nemo was kidnapped and his mom died. Come to think of it, that's every kid's worst nightmare! To this day, my daughter refuses to watch Finding Nemo - even if I start it after that scene.

I watched "The Little Engine That Could" with my son years ago, and he was scared witless by this character called the Nightmare Train. Who would put a Nightmare Train in a movie called The Little Engine that Could?! I thought maybe my son was the only one that was scared but I went to Amazon, and it's filled with complaints from parents who said their kids had nightmares for weeks!

What movies have your kids found too scary over the years?