Thanks to one and all for the responses. Seems like Camelot might be a bit much for my nephew, but it does sound like it's right up my alley; I will give it a try shortly. Will likely follow it up with Longbow.
Just try it out with him and see if he likes it, you can always move to something else if he doesn't.
Can anyone recommend any other kid-friendly adventure games with a text-parser? A historical or high-fantasy setting is a plus.
Infocom is the way to go then. You have both an Arthur game, and Shogun (based on Clavell's novel). I think they also had a 3rd game in that vein and using the same engine (they're like the Legend games, text with some still graphics; though you can turn off the graphics and simply play them as the old Infocom games) but I can't remember what it was. A Mind Forever Voyaging is more or less kid-friendly, but it's not historical/fantasy and it's quite serious in tone. Parser, but quite easy as a game. Stay the hell away from the Meretzky games otherwise! Your sister WILL skin you alive if she sees you showing him Leather Goddesses of Phobos (though it IS a great game). The original Zork trilogy might work - fantasy and parser, nothing too adult in there IIRC. Enchanter trilogy is harder, not sure whether this level of difficulty (Spellbreaker is HARD) would qualify as kid-friendly. IIRC Wishbringer was another good introductory Infocom game. Moving on to Legend, as Ms Bee suggested Gateway and Timequest should work - Timequest has a lot of pseudo-historical stuff in it, it's a great non-linear game, excellent parser (you can turn off the list of verbs/nouns that Seerated mentioned, I usually play without the lists). Eric The Unready might have too many "adult" jokes for your sister's liking. Stay the hell away from Meretzky's Spellcasting series! I think this covers most of the good old commercial text adventures.
Sierra's games have some good ones, but again beware the lewd jokes. Space Quest have a bit of that but they should be ok for a kid (mostly). King's Quest are alright but the parser (especially in the early ones) is poor, though he might enjoy the fairytale pot-pourri that Roberta had fun throwing together. The rest of the QFG series would be my recommendation; if it has to be parsers then definitely QFG2 original, at the very least.
Outside of the art-design, I was never a huge fan of Myst. In fairness, I haven't looked at the game in close to 20 years, but I remember thinking it was some boring ass shit (and it had some cringe-worthy FMV iirc). Was I just being young and dumb? Is Myst worth another gander?
If all you took from Myst was the FMV then yes you were young and dumb
It's a slow paced game for sure but that's personally something I
like. Exploration and puzzles are great. Riven is harder and supposedly better, but I like the first game the most.
Of course if you move into point and click then the list gets MUCH longer.