Contrary to what you might think, children sleep well with a certain level of noise, especially if it's the familiar sounds of the family doing whatever they usually do or the voices of people they trust. It can be very reassuring, especially for smaller children. That's the reason some parents leave bedroom doors open a small crack: You hear the baby/child, child hears you, everyone is reassured that everything is ok. (Note that some babies can be pretty noisy in their sleep, too: they sigh, snore, grunt, babble or moan... Can be terrifying or confusing at first, but you get used to your baby's typical sounds soon.)
Yet this is no free pass for shoddy sound isolation in the walls: While some noise is fine, I wouldn't want a child exposed to the sound effects of a typical Hollywood Blockbuster action or horror movie - screams, gunfire and explosions IMHO shouldn't be part of any child's dream soundtrack. I know a few parents that invest in a white noise machine to somewhat "even out" TV sounds or external noise.
And one day said child will be a teenager - unless you don't want to watch a presidential debate with a background choir of Heavy Metal, Rap or whatever, good insulation is a must. And while you can have one teenager wear headphones, a method to silence a bunch of giggling teenage girls hasn't been invented yet, afaik.
(I'm fully aware that construction methods and standard floor plans vary a lot over the world and you didn't say where you live, so work with whatever is available at your locale. )
