The family and I will be driving in Sunday. Where would you suggest we park? Keep in mind we'll have a four year old with us.
I'm assuming you are driving in down GW Parkway to 395/695 SE/SW Freeway (I think that's what it's called).
If so, I'd take the South Capital Street Exit, go straight past the park (it'll be on your right), move to the right lane, and turn right onto Potomac Ave SW, then a left on Half Street SW. You'll then be able to pick from lots on your left and right that are in the $10-$20 dollar range depending on what they are charging that day. You'll be entering the Park at the Home Plate entrance.
One word about Metro... kids love trains.

Emerging from the Half Street exit and looking into the Park from the CF gate is pretty cool. If the kid is on your shoulders, he would probably be psyched. Also, it would give you an extra 45 minutes to shake off that extra beer you had.
Also, you don't have to do SG. You could drive around the Beltway to the Greenbelt Station, which is *on* the Green line. No transfer, shorter train ride, etc. My boy, who is UMD now, rides the Green Line to the Park pretty frequently and thinks it's great.