Just in case anyone out there on the Internet would like to know, the current version of Minecraft runs quite happily (according to Mr 7 y.o.) on an iMac Core 2 Duo 2.4GHz 20-inch (Al). The iMac has 4GB of RAM and is running Mac OS X 10.11 “El Capitan”.
I am now the proud owner of a Pebble Smartwatch (I went for the “Classic” version). It seems to play nicely with my iPhone 5s. My favourite “watchface” at the moment is the built in “time as text” one (although I do think the iWatch one is cool).
I have installed the Pebble SDK and I am going to try to develop an app/watchface or two. Stay tuned.