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.

A budget option would be to paint the bricks with chalk paint. You can get different colours. Then some shelves for craft supplies in the alcove.

I would definitely get them full size single beds, whichever solution you go for. You could get them each their own loft bed with heaps of storage and a desk or pullout desk underneath each. There are lots of different variants of these and some would even help to make them each feel like they have their own room or at least space. If you would like bunk beds that can still be their own single beds later, you should look into 'Bunkers' as this is their trademark. They are Australian made and all pine as well so no nasty chemicals. Pretty sure they also have the loft bed options and desks that you could put underneath. They do bunks that can go at right angles to each other with storage underneath in the corner as well so you could put the beds in the corner opposite the wardrobe and desk near the wardrobe instead of under the window if you prefer this to their own loft and desk/storage underneath:-)

I would you for neutral walls and floor and add colour with accessories and toys. This means that the decor can be easily changed as they get older. Perhaps shelves to dispays their treasures, or decorate with some of their own artworks.