In general, a first person POV is where the narrator is a character and says
I pointed the gun at his head. I said, "Notice, I am taking the safety off."
He spit at me.
Stories have been told in 1P but each chapter from a different character, but only from one character at a time.
The 2nd person POV is where the narrator makes YOU the character in the story.
You run to the end of the hall, but the woman is already gone. You exit the building, and cannot see her anywhere. You return to your apartment door, wondering if you should even touch this package.
The 3rd person POV comes in multiple flavors, and is the most often used. In all cases, the narration is always "He did this, She did that" etc. Dialogue tags are always "he said" or "Cathy said" or "they shouted" etc. Never "I said", never "you said." Also, the narrator never addresses the reader directly; nothing like "I'm not telling you what Harry discovered."
3rd Person Omniscient means the narrator knows all the thoughts and feelings of every character, and can switch amongst them at will.
The biggest writing problem with 3PO is not cheating the reader. Once they know that the narrator knows everything, they expect the narrator to tell them anything important. This makes it very difficult for Larry to lie to Mary, without the reader knowing Larry lied to Mary.
For example, suppose Mary grew up with a single Mom, now deceased, and Larry is her actual Father. You can't really save that as a twist for the end, and pretend this is never on Larry's mind when he seeks out Mary and tries to help her. If you are telling us Larry's thoughts and feelings, it is unfair to hide the facts of his motivation.
Third Person Limited (3PL) means you tell the story from the POV of one character, or often one character per chapter. It is bad form to mix POV's within a chapter.
That is limiting, you are correct that you cannot skip for a paragraph to the stalker's POV if you are in Cindy's POV. You can write from her stalker's POV, but then you can't tell us how Cindy is feeling about something, or what she's thinking. That is cheating, you should be in 3PO all along, but then you have the problem with characters keeping secrets.
You also have this problem if you are in 3PL and switch to the secret-keeper's POV, again, once we are in the POV the reader expects any major secrets to be revealed (thought about).
But there is another switch-up; 3PL can be shallow or deep. A shallow 3PL still follows one character, and doesn't show what they cannot sense, but doesn't talk about their thoughts or feelings. It is like watching a movie: absent any voice-overs, we only SEE and HEAR the characters, we don't know what they are thinking unless they tell us.
That is how they can get away with following a character, working alone, that we only realize much later is a villain. In 3PL shallow we just watch them doing and saying things. That may not sound like good writing, but I'd say movies have enjoyed some measure of success as entertainment, so it can't be too bad.
Most 3PL is deep, meaning whichever character you are following, the narrator is privy to their thoughts, feelings, confusion, and entire mental state.
In general, even within 3PL, you should pick a lane and drive in it. Confused POV or sometimes omniscient, sometimes deep, sometimes shallow, is just disorienting to the reader. If you are a beginner without a track record of successful sales, agents and publishers will likely reject a book that tries to mix multiple flavors of 3PL. (They offer leeway to authors with a proven fan base, they can count on those sales. That's why bestselling authors can get away with doing stuff that would get a beginner rejected.)
For the OP specifically:
when the child leaves the room, it would be convenient if the reader kept with the conversation ...
...that would be helpful, I might also want to extend the scene
two people see the child walking into a building, and have a conversation about him ...
I would try very hard to eliminate these, more on that in a minute. Presuming it is impossible to sustain tension without scenes like this, I would indicate a scene change with three asterisks (***) centered on a line, followed by a POV change, to 3PL shallow. Meaning, pick some character (the guy on the phone, the driver of the char) and describe what they are doing as if in a movie, no thoughts, no feelings, no memories, no sensations, no description of the past of the character, just visuals and sound. Visualize the scene and pretend you can only see it through a camera. e.g., don't write "Jack was bored watching the building ..." That is a feeling. You can say,
Jack and Bill sat in the car. Bill was asleep, his hand on an expensive looking camera in his lap. Jack's head was drooping, his eyes closing for long seconds. As Marcus turned the corner, Jack suddenly grew alert and sat up, giving a back handed slap on the shoulder to Bill. Bill startled awake and automatically brought up the camera.
"That's the kid there," Jack said.
If you do this, it is very important to do it early. Find an excuse to do it in one of the first scene changes, like in the first ten pages or so (about 3% of the story). It is also something you should do often enough so the reader knows what it means, there's no hard rule, but once the reader sees one of these switches, they expect them throughout the book, and will anticipate them all the way into the final Act. In a way, these "asides" become like a story of their own and need their own arc, meaning these "outsiders" have a problem, it gets complicated, maybe it gets less complicated, then it gets resolved (they definitively succeed or fail).
So this is (IMO) not a technique you should use in just the first third or half of the story, that is probably a shortcut to exposition you should not be taking, because it will make the reader wonder where the outside POVs went.
If you have other scene changes within a chapter that STICK with the kids 3PL Deep POV, then in addition to the "***" you should signal these outsider 3PL Shallow POVs somehow.
I would do that by ensuring every one of them began with a name that is not the kid's name, i.e. the first word is a proper name like "Jack" or "Cheryl" or "Susan".
But when you talk about the MC, begin without a name, and use the 3PL Deep POV in the beginning (something that the reader quickly learns only applies to the kid). for example,
The smell of chili cooking made him realize he was hungry. He paused the cartoon on his phone and rose to investigate what was happening in the kitchen.
Both of those sentence depend on 3PL Deep; sensations and the reason he was rising. Versus 3PL Shallow:
Susan was leaned back in her chair, reading a folder. She closed it and sighed, then straightened up and picked up her phone, dialing a number on it, elbows on her desk. Richard answered her call.
"Susan? You got my report?"
"I did, and it's bullshit, Richie. Bullshit!"
"I know, don't pull the plug, not yet."
Let me make it clear this is not the only technique that will work; the point here is to devise a stylistic contrast between your 3PL Deep POV with the MC, and the 3PL Shallow POV with everyone else, so the reader never feels disoriented by the transitions.