Reporter: We give them the utmost trust. I'll be back later. Reporter: Leaving our precious children in their care. Once upon a time there was a pretty princess. Reporter: Most are kind, caring and... See More
Reporter: We give them the utmost trust. I'll be back later. Reporter: Leaving our precious children in their care. Once upon a time there was a pretty princess. Reporter: Most are kind, caring and loving but the horror stories about nannies. They're all over the news. She would poke the child be objects, scratch the child and scrape off skin and draw blood. Reporter: We set up our hidden cameras in east Rutherford, New Jersey. Jazelle is playing an abusive nanny. I'm not playing with you ava, put it down. Reporter: She's watching ava and her baby sister, who's actually a doll. What would you do if you saw this? Are you listening to me? I don't get paid enough for this. I wish you were never born. Reporter: At first, passersby clearly hear the verbal abuse. You know, I don't get paid enough for this. Reporter: But they keep on walking and don't say a word. Come on! I'm gonna beat you is what I'm gonna do. You need to start listening. Brat. Reporter: This man later says he didn't step in because the abuse wasn't physical. God, I wish you were never born. Reporter: This woman watches and almost gets involved before she turns around and walks away all because of something she says happened to her husband. He was punched in the face because a guy was beating his girlfriend up and he tried to intervene. Yeah, so. Pick it up! Reporter: This woman stops dead her tracks and just watches too. Look at me. You think this is a game? Reporter: Seems that people aren't sure when to intervene. So, we push it further. Did you slam it? Did you slam it? Reporter: That's when this woman moves in. Hey! That's no way to treat a little girl! I don't get paid enough for this, she's a brat. I don't care if she's a brat. You just knocked it out of her hands. Why don't you bend over and help, not kick them at her? I'm the one in charge she has to do what I say. Well you treat her decently. You treat people the way you want to be treated. She's a little brat, like her sister. Oh yeah, you do anything to her little sister, I am calling the police. Because this is hinging on abuse. Look, this is none of your business. Okay? Yes, it is my business. I'm a parent and I'm a grandparent and I wouldn't want my children treated like that. You have no right to treat this child like this. Reporter: We tell the nanny to storm off. You watch her then, you could have her. Do you know your mommy's phone number? 917 -- Reporter: She's actually calling one of our actors pretending to be the mom. Hi, is this Diana? Yes it is. Is everything okay? No, it's not. This woman is very mean to your daughter. She said your daughter is nothing but a rotten brat. I have a friend in the neighborhood. He'll be right there, his name's John. Reporter: That's John Quinones. It's a good thing I'm right next door! How are you ma'am? He's with us. Are you okay? You're shaking still. She's an actress. She's fine. Oh, sweetie. I was scared for you. I'm glad it was just pretend. Better to let other little girls know they have to know mommy's phone number. Reporter: This is our nanny, she's okay. I'm sorry. I know I gave you hell. It's okay. You stood up for her that was awesome. You were passionate. Oh definitely, I'm a mother. I will be a mother forever. Why you not looking at me when I talk to you? Look at me! Reporter: And now our nasty nanny is back at it again. Move faster! Reporter: The hurtful words are enough for this guy to step in. You said I don't get paid enough to do this. I don't get paid enough for this. Yeah but you can't be yelling at someone's kid like that. This is not your business. It kind of is, I mean you're sitting here screaming at a little girl and she just dropped some crayons. What's your name? Jazelle. I think I might have to call the cops. Does she hit you or anything? Ava! Who you calling? Sir! Rutherford police. There's a woman out here with a young child, she's screaming at her. Reporter: Of course the police already know we're here. It's time to break the scene. Reporter: It's "What would you do?" It's the TV show, "What would you do?" She's an actress. Holy crap. Reporter: After a few seconds he calms down. She said it's not your business. No, it is my business. If you're walking by somebody on the street and you might not know them but yeah, you can't let it go on. I don't get paid enough to watch you, you little brat. Reporter: Throughout the day we meet many people who do step in and confront our nanny. Get the crayons too, you little brat. What are you doing? How dare you treat her like that? She's a brat. It doesn't matter. You're teaching her to be a brat by being a brat yourself. Who are you to call that child an idiot? I'm her nanny and she is an idiot. You don't ever call a child an idiot. That's gonna ruin her for life. I hope you lose your job! Reporter: And what about the other men passing by? Well, along comes this father. She's stupid. I think actually you're stupid. How many times you gonna call this girl stupid? I'm a father, I don't know who you are or if you ever plan on having kids but I don't think you belong having kids. Is everything okay? She's a brat! Reporter: This woman calmly observes the nanny's ranting taking it all in. Can I help do the cleanup so she can have a break and you can too? I have two kids and I know they get frustrating sometimes. Reporter: You're very gentle in your approach. I have kids, they trained me well. I was just about to go tanning and I was making a phone call and I saw your daughter across the street with the nanny. Reporter: And then we meet Myla, who comes from a large family. She blows off an appointment to come to our little girl's rescue. Can you stay with her and I'll talk to jazelle? I don't feel comfortable giving my phone to her but I'll stay with her until you come. Reporter: When we tell our nanny to leave -- This is ridiculous. Come sit over here next to me. Reporter: She starts bonding with ava. How old are you, 6? How long has she been your nanny? Maybe when I was like zero. Zero? So she's been your nanny a long time? Has she always been this mean? Yeah. Do you tell your mommy? No, cause I'm too scared. My nanny might hurt me. Okay, let me tell you something. I have eight sisters. That means eight of little you walking around. And our mom and dad always tells us anybody is bad to you, comes up on the street, you have to tell your mom and dad. Promise you're going to tell your mom every time someone's mean to you? Yes. Pinky swear? Okay. Reporter: Now, guess who's around the corner. Mommy! Hi sweetie, are you okay? Yeah. Reporter: And so are we. Myla, come here please. You were wonderful. What a pleasure, meeting you. Are you okay? No. This is so embarrassing. Reporter: Are you kidding? You did the right thing. If this was real, the little girl would have been in trouble. I have eight sisters. Reporter: You don't have to spend this kind of time with a stranger. She's so cute though and sweet, I felt so bad. She was treating her like a dog, go fetch. Reporter: You say you see this? In Manhattan, all the time. It's always like nannies with kids, the parents aren't around so they don't know. As people, as new yorkers, we don't really say anything. You're a good actress. Thank you.
This transcript has been automatically generated and may not be 100% accurate.