My smallest nephew–a baby–and my smallest niece–four years old (which, she informs me, is “not a big girl yet.”) came over yesterday.
Whew, my niece hates the dog. She was in hysterical tears about him and no matter how much we reassured her, she cried the whole time she was in the house.
Fortunately, I have a big outside she can play in.
All the crying got the dog worked up and upset though, so he was shedding and panting and just… I don’t think Sonnyboy has ever met anyone terrified of him before. He didn’t know what to do.
The baby, on the other hand, thought the dog was great. He rubbed his feet all over the dog. He put his foot in the dog’s ear. He put his foot in the dog’s eye. He put his foot in the dog’s mouth. He tried to put the dog’s nose in his mouth. I tried to make the dog understand that he could go anywhere else in the house, but he seemed to love it.
Ha ha ha. Lord. In real life, don’t let your baby put his foot in a dog’s mouth. Even writing it, it seems very stupid. But it’s not like it was some kind of “put your head in the lion’s mouth” trick. The baby was sitting on my lap, kicking around, and the dog came over and seemed not to notice the barrage of baby feet. Or seemed to enjoy it.
But then! Then he snuck out and took off and I finally found him four doors down, attempting to enter the home of three girls and their mom.
Like he’d had his fill of my family and was ready to try out a family with less kicking and crying.
Tiny Cat Pants 
That is one adorable little girl.
She’s got my sympathy. When I was a toddler, a neighbor’s sheepdog knocked me down and sat on my chest (so I couldn’t breathe) and vociferously licked my face (so I managed to gasp any air in, it was mostly going to be dog slobber). I was blue and limp by the time someone rescued me.
So, yeah, for many years my reaction to dogs was like your niece’s — “giant, unpredictable, scary-strong thing that will try to kill me”. I grew out of it; now my reflexive greeting to big dogs is, “Awww, hello, sweetheart.”
But our culture makes it hard to grow out of that and does things to us that drags it out and makes it worse, by gaslighting us constantly about the real dangers of big, strong dogs. When a big strong dog is dashing towards me in utter determination to jump all over me and claw my skin off, some idiot yelling, “He’s friendly, he’s friendly!” does not help, because the dog’s friendly intentions do not change the fact that the dog will harm me with all the best intentions in the world. Hopefully people treat your niece’s fears as just and reasonable and don’t gaslight her about them — a four-year-old trying to be aware of danger is a good thing.