A couple of chairs in front of the bay window might look nice with the sofa facing them and separating the LR from the DR visually. On the other hand, will the TV be in this room? That would change things, and I'd be tempted to keep the sofa where it is if the TV will be on the opposite wall (where wall unit is shown).

Good suggestion from Grover to start collecting ideas of what you like. Based on the measurements given, I think the tried and true sofa with two chairs will work well. If you arrange as in the diagram below, you'll want to choose a sofa with dimensions of roughly 7'L X 3'D. Chairs can be about 3'X3'. A small variance should be fine. A really good thing to do is to tape out the measurements of the furniture you're considering on the floor to make sure they'll fit comfortably.

Is your TV above the fireplace? I think that is what I am seeing and if so I will assume this is your main room and where you view TV and spend family time? Your TV location really limits your furniture arrangement possibilities since it will be the focus of the space and it appears from the photos that the majority of the space in the room is down at the opposite end. Rather that a 3d rendering it would help more if you could draw out the floor plan of the room with measurements and show us that.

Is the living room fireplace a wood burning fireplace that you will actually use? Is the cover in front of it a solid fit that will keep any warm/cool air from heat/AC system from going up the chimney? Since the living room is open to much of the rest of the house, consider having the living room fireplace converted to a gas log fireplace. No "smoking up" the more formal living space. Put your more formal living room seating area in the front half of your living room. You could put a (round) table with chairs (that could be part of indoor/outdoor living and entertaining including the patio or, since it adjoins the kitchen, even be used as a family breakfast table) on the patio end of the living room between the fireplace and the door to the patio. Rather than create built ins in the formal living room, you might consider enlarging your door to your patio and centering it. Alternately, on the exterior wall nearest the kitchen, you could create a home office area for your computer for communicating and meal planning -- even to be able to hear the news while you're prepping a meal and, instead of built ins, add furnishings suitable to how you will use that space -- perhaps including a barrister bookcase. Is the family room fireplace is a wood burning fireplace that you will actually use? Is there a cover in front of it that is a solid fit that will keep any warm/cool air from the heat/AC system from going up the chimney? Since the family room can easily be closed off from the rest of the house, keep that fireplace as wood burning fireplace but do add a suitable cover to the firebox to protect against having a spark start a fire. In the family room, you could paint the beams the same color as the ceiling, however, unless it is damaged or there is another issue, consider keeping the paneling. Would put the tv in the family room on the wall between and below the two high windows opposite the fireplace. If the room is too dark, you could add light painted) colored thin wood (luan) panels to the wall behind the shelves without painting the walls ... or removable peel and stick wallpaper -- without damaging the value of the wood paneling and, potentially, damaging the resale value of your home. The base cabinets of the built ins could be perfect toy cabinets. You could also add doors to cover the shelving above and create yet more hidden storage. Is that brick flooring actually vinyl rather than brick? Unless you truly hate the current family room flooring, consider keeping it at least until your children are older so that the family room can be where the children play without worrying about damaging the flooring. Adding a camera to the family room could enable you to watch over young children from anywhere else in the house. Using the family room as a play area means you could focus on comfortable furniture there and put your more upscale furnishings in the living room and dining room where guests will more likely be entertained.