I wouldn't soundproof off a bedroom like that unless the house has "linked" smoke alarms - anything built in the last 5-10 years or so, one smoke alarm goes and they all go off, including the one inside that isolated bedroom.
Or if you have a Burglar and Fire alarm system with networked smoke alarms and Rate Of Rise detectors in the hall and each bedroom, if you don't have the smoke alarms with the built-in sounders you need to put a separate alarm sounder in that bedroom. So you don't have to chance the outside siren waking you up.
Get the commercial sounders with the strobe light if anyone in the house is hard of hearing (or stone deaf). And a bed-shaker - electric motor you screw to the box springs like a 1/2 HP Magic Fingers.
Hanging an afghan or comforter on the back of the door with a homemade over-the-door hook hanger (hide the screws on the top edge of the door where they won't be seen!) would soundproof a lot.
Or you hang a few moving blankets on a curtain rod over the door where you can easily sweep them aside. No Mattresses.
Get white foam stick-on weatherstripping for around the door jamb, and a removable foam sweep for the bottom of the door "As Seen On TV!" to block the sound coming around and under - between that and a big afghan-like pad it'll kill a lot of it.