First off, oh please.

That argument only holds up if you had spent the time you would have spent building the computer making money by working instead. By your own admission you wouldn't have. You would have done, what? Fixed a leak in bathroom sink, or painted your house, or cleaned up, or some other household chore? Talk about useless knowledge and skills, yet I bet you don't pay someone to do most or any of your housework, am I right?
Now, playing with your kid, fine, that's lost opportunity. But I'm sure not even the majority of that time would have been spent playing with your kid.
Secondly, I think you're still arguing the wrong point. Computers rule the work world. It's not like you're learning how to change the brakes in your car, which for most people does not supplement their work knowledge. Instead you have just lost a great opportunity to pull yourself ahead of everyone else in the market doing your job. I don't know the economic term for it, but I know the worth of that knowledge is far more than $100.
Thus, laziness. It's ok, call it opportunity cost if it makes you feel better.
