To open “Minecraft” you need to install the legacy Java SE 6 runtime

Every time I try to launch I get the message "To open “Minecraft” you need to install the legacy Java SE 6 runtime". I've installed the only currently supported version of Java for Mac and it still won't work. Simply unacceptable since my 8-year-old son just parted with his hard earned £18 for this software that does not work. What will fix it?