We've moved this to the U.S. Airways (pre-consolidation) forum so you can get more informed answers by longtime US flyers. /Moderator
Declare larger liquids. Medications, baby formula/food and breast milk are allowed in reasonable quantities exceeding three ounces, and they don’t have to be in the zip-top bag. Declare these items for inspection at the checkpoint. TSA officers may need to open them for additional screening.
You
could have issues with TSA security with bottles of water, as the TSA itself is pretty vague on this issue. It might be easier to buy bottles of water from shops once you're airside.
The stroller is usually gate checked.
The car seat issue might be answered here, but I'd suggest a look in the
Travel With Children forum.
Upgrade, not very likely at last minute for two adults and infant in FAA approved car seat.