just a couple of inches left would help it look better- don't like the couch squished against the molding either... centering on the molding directly above will be more obvious than across the room-- except if you're sitting directly in the center of the couch. but the distance gives a different perspective depending on where you're standing and it won't look perfectly centered on fireplace from most places in the room anyway...

You could use painters tape to mock up how they would look. I think they'd be fine side by side centered on the towel rack wall. My eye would ignore the towel rack since that's not actually decor.

I would hang one of the larger pictures back over the bed. I think I would do the shelves offset from each other instead of lined up because that would be more interesting looking. Before you put holes in the wall, put up some painters tape to represent the shelves to test the layout. I don't have pictures of childrens' rooms, but here is a picture from a home office that shows what I'm talking about. You don't have as much room so you would have two across from each other.

The pictures do not look approachable where you have them. It seems like it would be diffficult to appreciate them from a distance, even a short distance, due to their small size. I think I would pair the two together but place them vertically over the blue table, This way, one can get closer to admire the art. Similar to the red squares in this concept, what do you think?