OMG. The horror, the horror. You are not alone.
I have been there.
Take everything out of the room. Scrub the skirtings with Zoflora. Also the radiator. When it heats up in the autumn it wil scent the room. Wash the paintwork, use a magic eraser on light switches and sockets. Sprinkle bicarbonate of soda on the carpet. Hoover and dream of the day you can rip it up and burn it. If you already thought to lay a wooden floor, well done. 50 points to Gryffindor.
Wash all the bedding, including duvet and pillows, on a hot wash with Dettol laundry liquid in the mix.
Remove all clothes and shoes and wash. When dry hang in a scrubbed out clean wardrobe. Sprinkle bicarb in shoes surreptitiously while you clean then remove before putting away.
Remove all the many, many junk food wrappers, coke cans and dirty plates/cups/bowls, somehow there despite your long-established rule of “No food and drink upstairs, please.” Also all the many, many tissues/bits of loo roll dotted around. Summer colds, eh? Terrible affliction. Always use gloves.
Buy a scented reed thing from the White Company sale. Or M&S. Possibly even two. Or more. Place/hide them strategically around the room.
Open the window. Often. They will close it. Open it again. And again. Ideally keep it open ALL DAY when they are at school. Even in winter.
Imagine how the room will look when you reclaim it after they’ve gone. Or when you lose patience and simply BURN IT DOWN. Whichever comes first. Either will be an enormous improvement.
Good luck!