Calling the police is a threat of violence. It's done within the framework of the law, but that doesn't change what it is.
Um, what?
Any time that you called armed people to a situation, the threat of violence is present. This is a good example.
A threat of violence can also be present when a 6'5" hulk opens the door on all 5'3" of yours truly. I also have zero guarantee my neighbor isn't armed. If I'm going to confront a stranger, I would prefer law enforcement professionals be present.
Would I call on a crying/screaming child? Tough call. Kids scream and cry, sometimes a lot. Often you can tell they're happy screams, or angry screams, or a tantrum. But if you can't, or it actually sounds deeply wrong, you also don't want to stand by and then find out that kid ended up in a deep freezer...
Then again, interactions with police carry finite risk as well, especially if you are non-white.
Overall, if I *really* think/feel there is something wrong, I would value safety of a child over what's *most likely* an uncomfortable conversation for the parent. So I don't begrudge the OP making this choice. But it certainly isn't straightforward...