Hmm, I'm not really sure.....

Well, one reason is that killer children or demonic infants are supposed to be scary.

Or, like you said, they want something innocent and have people feel sympathy towards before the kid will most likely get murdered....

I like the kids of the movie *maybe cuz nightmare has nothing but them* xD

Kids are cute but they always have that really creepiness to them
You don't expect that the baby your brought into the world could be possessed
They are very unreadable I think.

Because take the Orphan for example it's a normal looking kid but it's trick she's evil ;D

Because it's realistic. Kids are EVERYWHERE!! :O

it makes it real ! because it would be kinda weird if there were just adults ?

they wanted to add a little something to the movie i think but i may be wrong!

Lets face it...little kids are scary! lol Just playing...I agree with the others on here....little kids represent innocent and hope....so when the child turns out to be evil...it just freaks everyone out.