I have never seen a "toddler bed" at WDW. I have seen, and used, the following:
1) the pack-n-play provided by WDW
2) the bed rails provided by WDW on a regular bed
3) the trundle bed at POR, which is a smaller-than-average mattress on a little rolling frame, that slides out from underneath one of the regular beds. It's only a few inches from the floor.
We've used all of them. IMHO, the best bet for a toddler is the bed rails on the regular bed (my DD used them just fine even though she was still in a crib at home.) The trundle bed would probably be fine, too...but for some reason I just didn't really like DD "sleeping on the floor" in the hotel room.
The bed rails seem to work just fine *however* they are one of those things that can be requested, but not guaranteed. One time, we couldn't get any.

Fortunately, we were in a room where the bed wasn't bolted to the floor so we could scoot it over to the wall, but... that was a bummer. My sister has some traveling bed rails that she bought from One Step Ahead. If bedrails are a must-have, you might want to bring your own.