A.S.
I know this isn't going to help you but prior to marriage and kids I lived with a roommate in an upstairs apartment. My downstairs neighbor constantly called the landlord to complain that we were "walking too hard" or that she could hear us speaking throughout the day, could hear our tv in the evening, etc. We were not being loud. We were living very respectfully and actually fairly quietly. She called the police several times until they told her that we were not breaking any laws and to please only call in an emergency (she would call when we weren't even home because she could "hear" us "moving"). Noises are amplified when you live on a lower level. If there is no soundproofing, you will hear pretty much every step they take and yes, it is just normal noises of living. I know it is annoying but thems the breaks, as they say, when you are in a rental, especially a bottom rental that apparently doesn't have noise barriers. Just think, in a few months your new baby will be keeping them up and disturbing them every time he cries. Hopefully they don't knock on your door to tell you to stop your baby from being a baby because someone in their house is taking a nap. I am sure the father is getting annoyed with you because you are asking him to stop his children from making noise which is like telling the wind not to blow and I am sure you are getting annoyed because, to you, they seem unusually loud. The only thing I can think of is to see if you can move and repaint yourself so you don't have to pay the $4000. Good luck.