I don't think I ever played an RPG before about 8 ish, I played a lot of other types of games though. Thexder, Breakout, Pong, Manic Miner, Maniac Mansion, Police Quest, Cat, Commander Keen, Midwinter etc. I grew up in the best era of gaming.
I think my first RPG was Eye of the Beholder 2 which I still think is one of the best things I've ever played. It was great for kids, some of my buddies and I loved it so much. It was extremely immersive, creepy without being scary, and it had some depth without being complex. In hindsight it wasn't perfect because it was actually too hard for beginners, requiring you to deliberately grind on early areas over and over just to survive the later stages of the game, otherwise you get 1 shotted by Beholders that cast crazy powerful spells like Power Word Death or whatever. Also there was a whole level that you couldn't rest in which was really brutal. Once you know where to go and how to beat it, you can whiz through that level in half hour, but with no maps or internet and not much gaming experience or skill, it was really hard, maybe too hard. (But not always winning first time is a good thing and rare in gaming nowadays.) Also, at the same time the game was quite shallow because melee characters just had a single melee attack, and spell casters didn't have many spells. I love mages but all they had was about several spells. Also there weren't many items in the game. As great as old classics are, some standards are higher today.
But still, I would recommend that game to anyone who wants a game to introduce kids to RPG's. It is perfect for them, especially if you can give some pointers. And if they like that, Eye of the Beholder 3 is good as well. It got a lot of hate because it was badly programmed and buggy and was released too early which wasn't common back then, but it is still a really good game. The first one is too but I find that to be too mazy and I hate mazes.
Also there are logical conclusions to this progression too. Like doing EOTB2+3, then going on to Lands of Lore, and then maybe more of those games (although I only liked the first one). And then there are lots of other games similar to that from that era. Although I would just pick a few of the best, and then skip ahead to Legend of Grimrock 1. The puzzles might be hard for kids but maybe they will like the challenge and they can always google for solutions. And then moving on to some bigger and deeper games with different perspectives, like Summoner, Neverwinter Nights, etc. Also the first Vampire The Masquerade was a cool game as well. Bloodlines gets all the love but the first one was more like a traditional RPG and was under rated.
Betrayal At Krondor is a really amazing game, but I didn't play that until much later. I really regret not playing it as a kid because it would have blown me away.