Definitely not B and C -- the garages. If you DO end up there the whole game by some stretch, it's a PITA to get out of.

I've been using the lot at 1323 South Capitol Street SW. It's ranged from $25 to $36 (which is what it is for Saturday) but it's right across the street from the stadium. You have to get to it from South Cap going the right direction, but you can easily take 395 to South Cap, then when you leave take 295 out.

Another favorite lot is The Yards. It's a bit more expensive, but it's also right across the street from the stadium on the other side. Pretty easy access out.

There are a few cheaper lots on the other side of M Street, but with two young kids I think you're better off being closer and paying the extra money. I definitely wouldn't take two little kids to a game via Metro, though.