Our neighbours think it is compulsory to talk loudly in the echo-ey hallway when entering and exiting their flat (or maybe only compulsory when living next to foreigners ;P ). We greatly improved the soundproofing of our door by installing a curtain track above and fitting a heavy, lined curtain with velcro down the sides to close off gaps. We reduced gaps with foam tape, and I made a door snake to go across the bottom. Also helps with air con efficiency.
how old are the kids? just saying that it could be temporary or the type of noise could change considerably over time (ie from crying, to scootering across the floor, to playing football indoors, etc)
You can make a door much better at keeping sound out by improving the sealing. Most doors have big gaps at the bottom....
The company below will sell to consumers the industrial grade additions to doors needed to make them better at sound proofing.
R&R Fire Material - 專業 隔音 防煙 防火