For a kid:
Find a paint coating you like, and get it on there ASAP, you're about to run out of time to keep up the car. I'd suggest Carpro Dlux for the trim too.
303 fabric protection ANYWHERE there is cloth, carpet, fabric.
Acquire a Bissell spot bot or little green machine for the inevitable fabric mishaps. It works for the house, too.
Folex fabric cleaner is a great pair with that machine as well. Cleans stains and doesn't re-attract dirt.
DIONO makes a great seat cover that is plush but firm enough to prevent the car seat from permanently marring and stretching the leather in the seats. They also make a great back of seat cover to prevent kick scuff marks on the seat directly in front of your child.
Take the seat out once a month to vacuum under it, and relax the leather on the seats and allow the foam to return to normal.
If the car seat and car has the LATCH system, use it. Less stress on the seat belt, and a more uniform compression of the vehicle's seat.
Get a good plastic, leather and vinyl coating from AG. Lots of options. Use it early and often.
Set the car seat into the vehicle's seat, adjust the vehicle's seat back and forth until it's set where you want it, then use some tape to cover the track grooves of the seat to keep the crumbs out of the tracks and grease. Clear packing tape works well and doesn't leave too much residue.
Buy aftermarket floor mats that you don't mind washing to cover the real mats. Those rubber weather mats are great for that.
Take the kid's shoes off in the car. It's a beating, no doubt, but shoes are the number one thing that my daughter tears up mom's car with.
Train your child that the car is not a kitchen on wheels. My wife let's ours eat in her car. Cheddar Goldfish and animal crackers are the bane of my existence. There is no eating in my car.
Get the a vacuum with the strongest suction you can afford, with a crevice tool (long thin suction attachment) and use it weekly. The cleaner you keep the car, the less mental fatigue you'll encounter and you'll be less likely to say, "screw it, good enough".
Keep toys in a cloth bin, and keep them soft. All toys can become projectiles. Hard plastic bins can get debris trapped underneath them and scuff up the leather.
Use only cups with lids that kids have to suck to get liquid out of, or you'll be sorry.
Wash your car as often as you can. Kids will touch the paint, try as you might to stop it, if they touch dirty paint, you've got scratches.
Lift your kids into their seat until you can't lift them any more, my 2 1/2 YO likes to climb in by herself, I cringe each time, and it's mommas car.
Whenever possible, use momma's car.
Expect some wear and tear, but defend against it with all your might.