The single most important consideration for your parents is going to be their comfort.
For starters they must have something comfortable to sleep on or they're going to spend every day achy and tired, wishing they were sleeping in their own beds. The older you get, the more precious sleep is. If they can't get a good, comfortable night's sleep, then they aren't going to enjoy camping at all. I know a number of old-time campers who have given up tenting in favour of their memory foam posture-pedic beds for the sakes of their old back bones.
I would recommend going all out and getting them a luxurious air mattress, something like the Coleman Durarest Queen double high:
Or a large Aero bed:
Or a collapsable bed:
Next item on the list would be a comfy chair, they will likely be spending a lot of time sitting down around camp, so make sure they have a nice place to sit.
As far as tents go, I think it's largely up to them, I know a lot of old school campers who still prefer their beastie canvas A-frame tents because it's what they are familiar with and what they're comfortable setting up. So unless you're the one who is going to set the tent up for them, maybe hold off on getting them a new tent until they decide they want to upgrade (after going out once or twice with you and seeing how nice your tent is for example).