I’m not seeing pkgman expose that information anywhere. If anyone has a secret parameter, let me know.
While we’re on the subject, pkgman search really could use a -b option. B for bare, where it just gives the package names with no flags, no short descriptions, no padding. Like ls -b -1.
Was looking for that information earlier also without finding it, maybe @BiPolar can help out here, he’s the pkgman guru 
You can get just the names by running the output of pkgman search through something like cut -c -40, but it doesn’t trim the blanks. However using awk ‘{print $1}’ as a filter works perfectly.
AFAIK, the “repo” files that pkgman uses do not currently contain information about package sizes, and no secret parameter to help us.
I’d like to have that info avaiable, and already have it in my (generally overly-ambitious, considering my skills) ToDo list for pkgman. At this pace, I might have something to test… around R2 
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You need to understand that while a game might look basic graphically, that doesn’t mean it will have low hardware requirements. Minecraft is still a modern game, poorly optimized, and it relies on OpenGL. Without a fast CPU and a GPU with 3D acceleration, it will run very poorly. Performance also isn’t the only problem with lacking or missing GPU drivers. You also need GPU drivers for things like:
-Vsync or Freesync (preventing screen tearing)
-Being able to select your monitor’s native resolution
-Connecting more than one monitor
I would suggest that you go with a “safe” option for your son, at least for now. If you’re looking for an open source OS, Debian’s “stable” release should work fine for most use-cases. Once he is more comfortable with computers, you can try and introduce Haiku to him, while making sure that he knows that Haiku is a work-in-progress.