I would suggest a large vertical non-linear painted design (e.g. cherry blossom tree) which could add some visual interest, be kid-friendly, and create a change of pace from framed linear artwork.

You could paint the hallway a solid color, or use a simple framed mirror in that space, but I wouldn't add anything small or patterned.

Too much stuff and the pictures are too small. You probably don't even notice them as you walk by; they turn into wallpaper or visual noise. Keep the small stuff for photo albums, for looking at during quiet times.