What is your question? If you want furniture layout advice we need photos of the space as well as a measured sketch (graph paper is best) showing how the spaces relate.

I think a nice low cabinet in that empty space for a few toys and a small rug .

bump

Ah, there you go. I used bed risers in my guest room to provide extra storage space under the bed. I would use rolling storage bins underneath the beds. Maybe a dresser between the beds instead of that useless cubby? Honestly, training them early to put their stuff away is key. At the end of the day, set a timer for 15 minutes for straightening the room. My son has been doing his own laundry since he was 10. Still needs cues to put his stuff away though! If you can, install a useful closet system (I think the rubbermaid brand is well-reviewed on CR) that‘s available at a local big-box retailer.