Dont understand
 

When I park the A6 in the garage I leave the key in the vehicle. Ny son is autistic and twice now I have buckled him in the back seat and by the time I get back to my door, he has locked all 4 doors. I dont see a way to change settings. How is someone on the backseat able to lock/control all the doors?

When I press the rear seat lock button, I notice that all four doors lock

So, your kids can lock you out of your car. Stupid idea.

Not if you keep the key with you. With the key left in the car can't your son, when he gets older, and anyone else for that matter start the car? I always keep my key on me, always have - can't imagine why anyone wouldn't, but that is your business.
Before we had these wonder keys and touchy door handles wasn't the remote able to lock or unlock just the front doors with one push of the button, then if you hit it a second time the rears? So maybe there is a way to isolate the rear locks from the front. The windows are, so I don't think it would be impossible. Maybe a call to Audi of America could answer the queston. Good luck.
Late thought: isn't there ususallly a button on the driver's door whereby you can lock out the other door lock buttons. It might be on the door (old school) not through the MMI. Just a thought - I'll look next time I use my car.

I'm pretty sure there's mechanical child safety locks for the rear doors but if engaged, it's to prevent the doors from being opened by a rear seat passenger (child) which won't solve the issue. Also, in the MMI you can set which doors will unlock with the keyfob or the touch handles but again, that won't address the issue either. My guess is the only solution is to have the key on your person. Which is also the safer option all around, I would think, for the reasons Bob pointed out.

When I park in the garage I leave the key in the car. No chance that I will misplace it or leave it on a table in the house. I never heard of a backseat passenger being able to lock the entire car especially when the key is inside. My son is autistic and likes to press buttons. I could also see a noraml young child flipping the switch. It's just bad design.

I think the child safety lock button from the driver's side solves this problem.

Disable the switches in the rear doors. You might have to unplug the electrical connector to the switches or cut the wire.

When the key/fob is in the car it is like your son (assuming he is in the car) has it on him and he can do the same things that having the key/fob in your pocket or in the console when you’re driving allow you to do. The key/fob does not detect what sort of human has it in their possession. The key/fob lets anyone near enough to it do what they want, since by having the key/fob they have the owner's permission, the car would assume. The car is not designed to step in and say "No son, you're too young." How would it know anything about any one touching the various buttons and controls?
In all seriousness I think this is a situation in which you have to think through what the technology means and perhaps alter your preferences for carrying/ not carrying and remembering/not remembering the key, since something bigger and more important is potentially at stake. End of sermon.

I am a bit puzzled here. Is that possible (by that I mean design wise) to lock all four doors by simply locking rear door? I thought only driver's door locks all four doors. Of course assuming all of that is done from inside a car.