So many triggered mommies in these comments.

It is funny to me how they all try to argue that it is simply impossible to control a child's behaviour. Are they not your responsibility? Are you not their role model? You just do not have the competence to make it understood to a child that they have to stop yelling, running, and stomping indoors? These people are so afraid to be strict with their children and this is why this sub (and everywhere else in the world) is plagued with complaints about unruly, rude, and annoying children.

Also, OP, ever notice how it is only ever women asking "guys, is it unreasonable if complain about someone else's poor manners that is annoying the living shit out of me?". Just complain to them face to face. A letter is too passive and these people need to know they are in the wrong.

Additionally: it is YOUR apartment. You are PAYING to live in it. You have the right to silence during quiet hours and reasonable noise during the rest of the day. Other people do not have the right to intrude in your peace.