Fibber,
I'm not sure what your Giant is worth locally if you try to sell it-- but craigslist.org is the easiest way to move a bike. Where I live, you couldn't sell a used kid's bike for more than $30, because WallyWord is selling new them for $70. Never mind that the Giant is a way higher quality ride-- it doesn't really work that way when you're selling. If you're buying, yeah, you pay $$ for quality.
Bikes are only a good deal moneywise if you buy them and ride them into the ground.
The question you have to ask yourself is, are you really going to miss the $135 that you paid for the Giant? That aint a lot of money AND your kid had a good summer of riding, so maybe the bike investment already paid off? Nothing wrong with keeping the bike around for the family to use. You could also give it away. Or you could sell it, but don't be upset if the buyers think $80 is too steep and try buying it for $40. That's all I'd give you (nothing personal, of course, but I'm a savy lowball bidder for used bikes)
Kudos of the newish Trek-- buying it was a good thing to do.