I don't know of any specific apps like this (I vaguely seem to recall having seen something for iOS in the past, but I don't remember any details), but as a quick first pass, you could set up a webserver on the Pi, with CGI scripts (or similar) to trigger whatever hardware functions you wanted. Then they could just go to the Pi's website (on your local WiFi network), to push any button, or buttons, that you set up. As a bonus, on iOS at least, in Safari you can select "Share menu -> Add to Home Screen" to give them something that looks like an app to run. Perhaps Android can do similar. The webpage on the Pi would also be controllable from any PC's/Macs in the house. And, likely, any app you do find for remote access will expect, or at least be compatible with, sending URLs via http (or https), so you'll have already laid the groundwork for that. Plus, with all the smarts in the webserver, you can tinker with it, updating at will, without them having to re-download new apps. It's a start, anyway.