Nice of you to put such specific consideration into their needs! I would think that timber blinds or white timber blinds (venetians, which are much cheaper than shutters, but can look just as good. Because they are easy to clean, but mostly because teens tend to hide out in their room, sometimes this turns into a cave and they don't really care about opening the blinds each morning, also, they are particularly concerned about their privacy and sense of space, having blinds means they won't ever look messy and the most you have to do is open them up, and if they are tilted a Articular way, they still provide them a private blockout while letting the directional light in. I would also do a timber floor for a both, as they are likely to traipse in wearing shoes that of course are full of stuff from their adventures. They play sports, may spend time outdoors and timber floors scan be easily cleaned, carpets are ruined. As for a girl, if she has different interests, or spends time in her room often, a carpet rug on a timber floor. Rugs can personalize a space, so they have some saying the decor, and easily replaced, maintaining your timber floors beneath, while providing warmth and comfort. Even if covered mostly by furniture, the edge of a rug underfoot will feel more comfortable than not.

We're building 4x3.5 Two with ensuite and the others with a jack and Jill. Life has changed unfortunately and now everyone in the household works, kids stay at home longer and ageing parents sometimes move in for care. At the moment we have a 3 x 1 and really the only thing we really want for is a second toilet, which growing up we had because we had the one out the back with the red backs.

can put in skylight if you need more light ? more lights ? or build your fake wall a bit in and only high enough to hide the windows but not all the way up to the ceiling so you let in light from your 3 windows?

I think you need to soften the look and colour of the bed by mixing white and cream bed linen maybe with a little soft grey accents. I would go for a variety of textures in the textile as apposed to any outrageous colour and layer the look with throws and cushions. A mirror long and sleek would look great behind bed and a tall floor lamp and some greenery eg: potted palm in the corner by the window. Good Luck!