Oh, this is probably our favorite thing to do at WDW! For sure our fav out of park thing to do! The Boardwalk is the way to go in my opinion, though I hqven't tried them at WL. We did ride them at the Poly and it was fun and very pretty but just not the same. Plus, that location is only open until 5:00 and riding in the evening is not to be missed.
You have a couple of options--the two seater is actually one seat across the bike itself, with a basket type thing across the front which seats two little people with seatbelts (and access to the bell--very important and soooo obnoxiious when you are four!) To ride in a seat with the pedals you have to be tall enough to reach them and DD8 was not--she is about 50' tall. She did fit perfectly fine on the seat between DH and I while we pedalled and I am on the pooh sized end of things. So, for the 5 of us we rented a two seater the first time we went. Second time we had MIL and FIL there too and got the four seater--let me say it's a bit hard to get that baby up a hill! DH had to get out and push!
Lots of fun and not to be missed.