I don't know anything about child seats, but I would guess from POP to Boardwalk would cost from $10-$15. I'm sure others will have more info for you.
It would be easier to get to the Boardwalk area from either Epcot or the Disney Studios. You can exit Epcot at the International Gateway which is near the United Kingdom. It is maybe a 10 minute walk to the Boardwalk from there. From both the International Gateway and the Disney Studios you can also take the Friendship boats. They run all day between the two parks stopping along the way at the Beach/Yacht clubs, Boardwalk, and Swan/Dolphin. Slow but a nice relaxing ride.
You can also walk to the Boardwalk from the Studios in about 15-20 minutes, but with a small child you may not want to do this.