I'm so happy to see a couch that is not flush to the ground:) good to see the carpet under the sofa, feels less heavy, room looks bigger etc. I agree with bigreader, big art work, big rug, some panels at the window, no more cushions though

Yes, a standing lamp would be a nice addition. I think you just need to add some things that are at eye level. Look at your photos and see where the room is not "dressed". The bottom half of room has a lot of interest but not top half. So some things that draw the eye upward (wall decor, curtains etc) will help balance the look. Good luck.

I like the idea of a navy rug with perhaps some lighter blue and white/beige and if possible get one that is textured to add pizazz to the rom and off set your lovely light blue seating then maybe a throw rug with similar colour and a textured vibe to 'throw on the lounge, you could also pick a couple textured cushions and take away the flatter style material ones and decide how to mix and match them with the new textured, then as suggested maybe a large abstract painting behind the couch with the navy, white and even a touch of another pop of colour that is warmer eg, orange, hot pink and white just something that really pops and yet looks good with the furnishings, I agree any rig should be much larger than the one you have, in the corner by the window a large free standing pot or vast that would fill the area with interest as well, up to you if you put a plant or use it just for decoration.

One tip is also the small pic you have on the wall looks overwhelmed in that large area, a much bigger one is better idea maybe move the smaller one to a smaller space or area where it doesn't look so 'lost'. good luck

Sorry about my spelling folks, bad head today it should read rug not rig and also large pot or VASE not vast...oh boy, think I'll call it a day!

Love the little one playing in the photo, because you do have a little one I would suggest a couple of timber side tables instead of a coffee table, no glass please.

Pull your patterned rug back towards both lounges, it will add character and is much better to have this rather than a coffee table obstructing the open space to play in.

A much bigger wall hanging and a standard lamp,

being a rental you most likely can't hang curtains? Pretty Lounges.

I'd go for patterned fabric for cushions - get more than one, that go with the couch as is, then use the colours in the fabric for curtains, rugs and to put up a large artwork behind. You could start with an artwork, that has at least tinges of the blue in it (not huge amounts, just some) then make cushions and rug/curtain to go with both art and couch. With young-ish children, avoid really light colours and remember some patterns can hide all sorts of minor disaster.