SmartSim is a really impressive project for a 16-year-old! I've been playing with it a bit, but I find the UI somewhat cumbersome. (I'm too used to standard UI components like multiple select and copy-paste that I haven't found in SmartSim.)
Logisim has some big plus as far as simulating computers. Perhaps most useful are that wires can be buses up to 32 bits wide and basic parts like chip I/Os, gates and adders can deal with buses with single wire connections. Logisim also has much richer built-in I/O devices like keyboard and text output.
I forgot to include the link for Logisim. You want to use the original and not the various "reboots" that are floating around.
http://www.cburch.com/logisim/I can't help much with what's out there in the way of kits; I learned all this 45 years ago in the heyday of Vacuum Tube (Valve) radio kits and the dawn of the hobby computer revolution.
Googling for "monthly electronic kits for children" turned up a few interesting sources, but most seem rather pricey.
Another good online site for self-learning is
http://www.play-hookey.com/--Mark