I like the concrete bloc walls. They are becoming quite fashionable today. I'd keep them in the natural concrete color or if they have been painted, a color paint that resembles concrete. The concrete walls give a hard and cold impression so I'd choose wood flooring for the floors. The combination of a warm wood on the floor and hard, cold concrete on the walls is a nice one. Brighten the space with colorful modern art.

I see a window there... good! Is it facing north or which direction is it? Are the doors & trim throughout the house all dark brown? Light doors & trim would help alot. Lighter happy paint colors, such as butter yellow or limon walls would bring a sunny warm fresh feel to the room. Try some punchy colored feature walls as shown in Manon's above photo (thanks Manon). Get new furniture & some fun colored art & accessories, everything's looking very dated. Better lighting & a tall floor lamp that spreads light across the ceiling or corner. Lighter drapes (with grommets) down to the floor would help OR Roman shades or 2" faux wood blinds covering just the window would modernize the space and give you more wall space. New flooring in updated colors such as white-washed gray driftwood. Designate areas or stations in the room for different activities, such as floor games in one corner with colorful interlocking foam floor mats, comfie seating (bean-bag chairs) for movies in another area, table & chairs for table activities (coloring/puzzles/games/crafts), etc. Are your children boys or girls? That'll make a difference in what you choose and how you put it all together.

The market is full of tropical/botanical art at the moment which lend themselves well to tropical or beachy looks. The look you are after calls for natural textures with a soft colour palette, think linen bedding and layer it up with throws and cushions for texture and pops of colour. In my opinion drapes are not a good option in your room as the ceiling slopes to just above your window at a certain point. This would dictate the height at which your drapes hang and, in this instance would be too low. To soften the windows, I would have a tall palm or Fiddle Leaf Fig tree in wicker basket. It also looks like you have the space for a chair.

A lovely rug on the floor would be nice. You seem to have some space, yet your table is pushed against the wall. You could consider a bigger table for centre of the room or making it into an office / desk / book shelf / comfy chair. You have lovely sun coming in. Even a bean bag and a small coffee table could make a difference. A medium sized plant would also look great. Set a budget and stick to it, re-purpose things if you can and / or buy second hand so you don't break the bank. I love the little air vent on the floor it looks like a little mouse house. Maybe you can get some removable stickers and make an art piece or at least a little mouse running to the hole.