@SteelSteve - Welcome. SMB/SOHO integrator here. I'd implore you put aside other suggestions for the moment and consider the following.
At a high level, I can tell you almost straight away you want to be broadcasting wifi from at least two, maybe three, equidistant places across the house, in order to ensure proper signal quality for all devices. That said, I would choose gear based on how much, if any, wired backbone there is, and/or how simple you want the solution to be:
1) If there is ethernet or TV coaxial cable (for use with
MoCa) in the house, then I would skip the consumer gear altogether and run SMB-grade, discrete components. Properly installed, neither you nor your parents will ever have to touch it again (aside from configuration changes). Help calls should also be minimal, likely zero. The setup would consist of a wired router/switch (example:
Ubiquiti EdgeRouter X - $60) and two or three controller-based access points (ex:
Cisco CBW140AC ceiling unit ($110 ea) or
CBW145AC wall-plate unit ($150 ea). If the backbone is ethernet, you can power the APs via PoE, centrally with a PoE switch or injectors, or locally with injectors or AC plugs. If backbone is TV coaxial, then you'll need
MoCa-compatible splitters ($10-20 ea) in place of any non-compatible ones, plus one
MoCa 2.5 adapter ($60 ea.) per wired equipment location. APs will need to be powered locally by injector or AC plug. Although this is a more complex setup, with added steps and gear if MoCa is involved, the end result will be a network that functions more like an appliance and less like a toy (think: hot water heater, HVAC system, etc).
2) If you can't create any wired backbone, and/or or just want to K.I.S.S. at all costs, the just go consumer wifi mesh, namely
Eero (any hardware version). Why Eero? Unlike most consumer router firmwares with meshing added after-the-fact (ex: Asus AiMesh, Netgear Orbi, etc.), Eero 1) purpose-built,
actual mesh, 2) uses QoS that actually works (to keep all internet activity snappy and buffer-free, regardless of what anyone else is doing on the network) and 3) can alter radio roles and channel usage in real-time to optimize traffic flow and keep all mesh links in-tact. No other product in the segment (or out of it, for that matter) does any of those as well, let alone all of them, and together they make a huge difference in the "it just works" factor. Eero also has dead-simple software that I've found to me much more stable, on average, than your typical Asus, Netgear or TP-Link all-in-ones. Granted, you can augment much of that last issue with stuff like Merlin for Asus, but it's one less thing to have to monkey with when picking a product that just works, non-stop, straight out of the box.
So that's your action flow to solve their issues once and for all. Any questions, feel free.