If this was mine, I'd go with simplicity and do the walls in white subway tile, and on the floor, I would probably put in hex or penny rounds. You could add color if you wanted by putting in a blue pencil running through the subway tile maybe a third of the way down from the top, or at the very top like you have something now. I think subway tile is really a classic, clean look, and it will save you money. I assume you're taking down the rest of the tile that's on the wall? Alternatively, you could go with a porcelain tile on the floor and walls-- big squares on the diagonal on the floor, and big squares on the wall. Check out Crossville Porcelain, especially their Empire series; they have polished and unpolished tiles in lots of configurations that look terrific, in all kinds of colors.

go with a classic neutral like the white subway tile or large scale porcelain tile for less grout to get gross. then you can change styles when they want something more mature than minions and mindcraft.
make sure the floor tile has some texture for traction when wet. My son soaks the floor more as teen showering than in the splashy baby stage.