I went alone in Seattle last year. My brother in law lives in Vancouver so whenever my wife wants to go visit i make sure she lines it up with when Jays are in Seattle. I've got 2 boys, 5 & 3 so last year they would have been 4 & 2. I've brought the oldest before but it's really just a giant waste of money because he could care less about the game so I went by myself.
Bought the cheapest ticket on stubhub or a single from the scalpers on the street and then just sat anywhere. The best strategy is to stand at the top of one of sections behind home plate. Scope out some empty seats and after a few innings with nobody sitting in them it's yours. Seattle is a great place to watch a baseball game. Great beer selection too.