I agree to an extent, woof, and love 452s, but have you handled a T/C R55 or its predecessor, the semi-auto .22 (no model designation)? These are all-steel-receiver works of art. Good stuff, maynard. True that not as classic and not as accurate as a turnbolt like an Anschutz or Sako let's say, but as semi-autos go, the T/C is really nice. Not necessarily a contradiction - just *a* category of rifle - the "best semi-auto .22" is a valid question, I think. I'd reiterate the idea "forget it and get a CZ 452 or an Anschutz or Cooper" if he can be convinced, but at present, the man wants a semi-auto, and so man should get a semi-auto.
I think at 6'4", the main questions are:
1. What's the length of pull NOW, and
2. Is there gonna be any way to change the length of pull for the kiddos to shoot?
My take on that is this: If it was just for you only, at that height and arm's length, you'd want something in the 13.5"-14.0" range (LOP). BUT, since (a) you want the kiddos to be able to shoot it later, presumably starting at a fairly young age, and (b) since even having long arms, you ARE still able to shoot a .22 in a very short LOP config quite happily, due to there not being much recoil in a rimfire, so scope-eye is not an issue; or alternatively, if you shoot iron sights -- because of those two things, I'd lean heavily toward grabbing a SHORT LOP rifle, so that the kids can get after it at 10 or 11 instead of 13 or 14 years. That means I'd look for LOPs in the 12.0-12.5" range, or at the most 13.0". I do not know what LOP the T/C has - you'll want to research that and on other models, too.
Another thing - if you look at OLD semi-auto rimfires in the used markets - 1960s and older, or maybe up into the early 1970s even. Almost all of the old ones are very good rifles with STEEL receivers. Any number of old Remingtons, Mossbergs, Winchesters, Savages, and Stephens rifles. Some have dovetails for rings and some don't. There are a LOT of old semi-auto Mossberg models, which tend to be very good.