The little girl in Dawn of the Dead '04
was bad, no doubt.
Any violent situation (hypothetical or real) pitting an adult versus a child is nauseating on a number of levels. It tugs at parental instincts for sure, even for those without children. But it also chafes against any sense of fair play at all - physically it simply is no contest.
That said, the little girl in Dawn of the Dead '04 was the neighbor's little girl. So while horrifying on one level, it doesn't reach the parental system shock seen in the Dead Island game trailer
for instance. Despite being seen from a 3rd person camera, and not having the advantages of using live actors, those scenes put the viewers' perspective & emotions smack into the parental point of view. Chilling.
The zombie boy Jim kills in 28 Days Later (couldn't find the scene on YouTube) is also pretty scary. It even features the one zombie who speaks - saying "I hate you." That's also something most parents will identify with, and it's haunting.
But, I think the little girl
in the debut episode of Walking Dead was the worst I have seen in terms of frightening female zombies. That scene excelled at showing Rick Grimes' reaction to this particular zombie. Though she is undead, Rick hasn't really come to terms with that yet. She's also still a little girl in her slippers who Rick just witnessed picking up her teddy bear, wandering aimlessly amongst the carnage left derelict around them. He thinks she's probably distressed and nearly out of her mind. He is almost whispering to her in calming, gentle fatherly tones. To cap it off, the scene doesn't have the "gotcha" zombie attack moment - it's all a slow build of fear, disgust, dread, and confusion followed by the crushing, cruel reality of survival all rolled into one.
I think hands down the little girl scenes are the worst. Take sugar & spice and everything nice, at an age where they possess clarity of intention and unending precociousness, and then twist it nearly beyond all recognition. We end up seeing what we want to see, not necessarily what is actually there.
I really like the zombie movies that get in your head.