i am bring a first time hunter deer hunting. he is 12 years old.
what would be better for them to shoot a 243. or 30-30?

Before shooting any of those guns try him out on a .22. See if he can hit the target at a hundred yards and slowly move it back.

If he continues to progress put him on the .243. If not the 30-30 is probably the one. Let him choose the correct one buy his proficientcy...

I'm assuming he's already proficiant at shooting at the range?

If so, I don't really think it matters. A 30-30 lever gun might be more fun for him, but my own son is much better with a bolt gun. They tend to take the rifle out of shoulder placement when working a lever action.

I will defer to others who are more experienced with deer hunting with centerfire rifles, but it seems to me that the decision would be based on where you plan to hunt. If its woodland where shots may be measured in feet instead of yards, I would think a 30-30 with open sights would be best. If its gonna be open terrain, then wouldn't a scoped 243 be better?

when i was tweleve all i got to carry was a bag of cheetos... but hmm

Depends on what you are going for, and what he can handle. (assuming it is first time hunter and not first time shooter) My first deer rifle wasa Marlin lever in 44mag, only because it was what my father could afford that year. The next year it was a 6mm Rem that he borrowed (and had to buy since I got my first buck with it) But since I was 10 I had been shooting his M14 and Garand for target and could handle anything under heavy magnums so I had also used his and my brothers rifles as well. My rifles were more a case of what I could handle carrying and lugging up the mountain and into the stand, not the caliber of the weapon.
But what your answer comes down to is what he can handle himself.

Ideally, if you had, or could borrow, both rifles and have him try some shooting from each, you could see what is best for him right now and go with that.

When I was 9 my dad shoved a 8 MM in my arms.Said if I come home without a deer I get my butt beat. He gave me one bullit. I came home empty handed and got gun butt beat over the head.

Ouch!

Mine figured if I didn't get my animal, HE didn't teach me well enough.
(of course "teach" is subjective. I used to have to play "hound dog" when I was first hunting with him. ie "you go over there and walk back and forth thru all of that brush and try to flush any rabbits over toward us","go run up and get my birds for me")

I would go with a youth model .243. Very dependable and fun to shoot. Just be sure that you get a rifle that is sized to the size of the hunter.

Both are good deer cartridges.

For a beginner, I think a bolt action .243 would safer. I think kids may have more trouble letting the hammer down,on a 30.30,after loading up or not getting a shot. That hammer could slip out from under their small thumb, causing an accident.

I also think that some kids may have more trouble working the lever as opposed to a bolt.