Not a grandkid, but I started my daughter off on a Contender carbine, once I found she was left eye dominant. She's right handed, and does everything right handed, but she's left eye dominant. I would wait and see which eye dominance your grandson settles down with before you jump to conclusions about right or left shooter.
Like you, I have a whole safe full of right handed rifles and shotguns, and a kid that shoots lefty...
Irony, that.
I got my daughter a 22 lr bbl and a 6.8 spc bbl. She struggled with the Contender as a shooting platform. She had a great deal of trouble opening the action, even with a "cheater bar" attached to the lever, she couldn't do it without quite a struggle. The frame in question was a pre-easy open frame, which is the only type I have. My daughter has smallish hands, and just doesn't have the hand strength required. The other issues were, first, she would forget to cock the hammer. Not every time, but often enough. Then she'd have to struggle with the lever again to recock the mechanism. Sometimes she would also forget to switch the hammer block off on the hammer spur. The resulting "CLACK" on the frame would confuse her and we'd start the whole process of cocking it all over again. Really not that big a deal at the range, but a big deal when either small game or deer hunting.
The last issue was the weight. The barrels I bought were "current" production barrels, which are bull barrel profile. They are heavy. The over all result is a rifle that is pretty nose heavy for it's over all weight, and it can be harder for smaller folks (kids) to hold it up, with all that weight sitting out at the muzzle. The best solution to this problem was presented by Ladobe, who kindly sold me a very light weight 6x47 bbl for her. That made all the difference in the world as far as balance went. She had no trouble holding that up. It's 18" long and a super featherweight profile.
On stock sets, I don't think you asked, but I couldn't find a "youth" stock anywhere. I ended up deciding to just get a full size stock and have it cut down. I bought her a walnut stock set to start with, but when it arrived, it looked too good to turn loose with a kid, so I bought a synthetic set for her. I still have the walnut set, and use it for me. I had the buttstock cut down and a pad put on it by a local gunsmith here in Phoenix. He did a good job. Stock fit is very important to new shooters, kids, etc, and I think it gets overlooked far too often. Make sure whatever you pick to start with, that the stock fits the shooter now. Not something "they'll grow into". One of the advantages of going Contender is the buttstocks are relatively cheap. I just wish the comb height on the buttstocks were made taller for scoped barrels. I consider that a design flaw.
I have since bought her a lefty bolt action Savage 22lr for plinking and small game. She likes that much better, and it's easier for her to manipulate all the functions of the rifle, load, cock, safety, etc. Plus it's lighter in weight than the 22lr bull barrel on a Contender frame.
The same barrels USUALLY fit both frames. All 3 of my "current production" barrels lock up and fire on both my pre-easy open frames with no problems. That may not always be the case. You may have to stone the locking lugs to get a particular barrel to fit a particular frame. You can also get new lugs from TC. The only barrel I have that gave me any issues was an earlier production 17 Rem carbine bbl. I had to swap the lugs on that barrel to a new style lug, but then it worked fine after the swap.
The buttstocks are different. G2 will not interchange with earlier frames, you are correct. The forearms however, are dependent on which barrel you are using, not the frame it's going on.
Any other questions, just let me know here or by PM.
