I have an 8" Global chef's knife that can be found for $100 or so (I think it's model G2 or G-2?). I have also heard very good things about Shun (the VG-10 steel version). That can probably be found in the same price range as the Global but will likely be a little more.
Just as an idea for you, if they BOTH like to cook WITH each other, maybe get them EACH one of these chef's knives. Or get one a chef's knife and the other a santoku so they have a little variety. This may just be me, but pretty much the only "good" knife I use in the kitchen is the chef's knife. I have a really old, cheap serrated smaller one if I need it. I never use a paring knife, but others do. It depends on the tasks your parents will need to accomplish. Basically what I'm saying is start with the king of the kitchen in the chef's knife, and you could always build from there for birthdays, anniversary, next Christmas, etc.
I would agree with the other opinions that you don't need to buy a whole set, as my post suggests. You can get a $5 bread knife and have it cut and last just as long as a $50 bread knife. Same goes for steak knives IMO. Personally I would never use a cleaver, kitchen shears, cheese knife, etc. that you may find in a larger set. But again, just my opinion.