It sucks living in an upstairs apartment with kids. Bad enough getting them up to the apartment with whatever you're carrying, worse having bad neighbors below.

Have you tried introducing yourself, or maybe leaving an apology/explanation note with contact information. Downstairs it probably feels like they are screaming into the void whenever there is noise, they might have a kid trying to take a nap or something that you can compromise about. There also might be certain noises that seem small to you but are more irritating to them.

Honestly, the only good downstairs (or upstairs) neighbor I ever had was a mostly deaf older woman.