Do you have a gamer at home? It's hard to get my kids of their consoles these days, so I think they're going to love this Gaming in Progress A5 Lightbox. It's reduced to just £7.49 at Amazon, with free delivery if you have Prime, so I think I might have to get them one each!
This is an A% sized illuminated sign reading 'Gaming In Progress' so your kids can tell everyone that's they're mid-game and not to be disturbed! It lights up in green when switched on, and can be powered either by USB or with 3 x AA batteries, which are not included.
The sign is chunky enough to stand on it's own, maybe on the desk next to that gaming pc or console, or alternatively it can be hung on a door.
There's free delivery for Amazon Prime members, or it's free on orders of £20 or more if you don't have Prime.