It seems that your attic has a high ceiling. You may turn this into a loft which will divide the room into sleeping, study, and play areas. This sample that I’ve got is good for more than 1 kid. Try to look more at this link: http://www.kidsomania.com/15-cool-design-ideas-for-an-attic-kids-room/
I highly recommend you to install an egress window for the safety of your kids. Make sure this window is large enough which allows the kids to escape in case of fire and also permits the rescuers to enter into it. I have read that there are criteria provided by International Residential Code (IRC). The minimum width of the opening should be 20 inches, the height opening should be at lower than 24 inches. Allowable maximum height above the floor is 44 inches. You have to compute it and make sure the measurements are proportion.
Well from what I have learned, make sure to paint the walls and ceilings white to make look airy and spacious. Use more lighter shades for windows and carpets then try to make color combination simpler to be visually refreshing. Lighter shades tend visually set off angled walls in a much dramatic fashion. One of the most critical elements in designing the kids room is the right color to choose. :)
I've read a lot about color psychology and you should definitely choose one that will be good for your children. I always opt for commercial painters and let them help me with this job.