My son recently purchased a great cube system/shelving with canvas cubes at Costco. They have two small kids, so I understand your situation. Diapers and wipes do need to be close by because it's often difficult to remove one child to clean up and watch the other at the same time. I saw this line of bookshelves in my local Pottery Barn Kids and think it's clever, but does require wall anchors for climbing kids. https://www.potterybarnkids.com/products/madison-bookrack-tower/?pkey=call-study-storage-furniture&isx=0.0.1221 For things you need to access, but don't want the kids to do so, would be a large storage ottoman on casters to be your coffee table. The lid on mine is too heavy for my grandchildren to open, but they love to hang out on the ottoman and are allowed, with a tray on top, to snack on it.

Needs to be non-flammable, you can google the table of clearances around fireplaces. I can't tell what your exact thickness is, but you could build it out with masonry (another layer of bricks, for example), steel studs and durock facing for the tile, a stack of durock sheets to match the thickness if it's thinner, a sheet of stone, maybe even wire lath and mortar. Whatever you plan to do I'd check it out with your building inspector before proceeding, they tend to have firm ideas about fireplaces.

Btw, in my second comment pics above - just was showing the darker tan cushion color - a couple of examples. Was not necessarily suggesting lime green pillows, etc. Although you do have a range of colors you could incorporate/with various possibilities I would go with your core colors, the tan and black - and use mostly tan on cushions in seating areas, with some white/off white on cushions where it might be possible - as well as some throw cushions with white, black or both + terra Cotta accents, as I showed in my initial comment. Just to again show general idea and some black and white accenting. and lots of terra-cotta ideas above. And I think a shade of blue on umbrellas could be nice.

Remove the mirro it reflects nothing worth seeing twice . I do not like all the stuff on shelves in kitchens but if this is the look you love then get piece of art thay would look great in the kitchendecor and put it on the mantel to beging with then candles more like the ones in the kitchen only a bit taller and bigger around . IMO the Edison bulbs in all the fitures are casting a yellow glow on everything IMO not falttering but you need to decide if that yellow glow is what is making your kitchen feel warm along with the kitchen wall color, which I bet if you changed all your bulbs to LED 4000K ,would look very different too.