I've been wondering the same. I guess it all boils down to a length/height thing. How long is the toddler mattress you're considering? Is it one of the convertible cot type toddler beds? One of my friends has her 23 month old toddler in a toddler bed made from the convertible cot. She says her DD has really only got a couple more months in it before she'll need to shift into a regular single bed - as she's growing quite long. My DS is really small for his age so I guess we'll get away with this for longer than most. They have only a 2bd house and bubba number 3 is on it's way soon. Something they are considering is a single bed that stores beneath another single bed. Not quite a trundle but the same idea. I think the mattress on both beds is the same thickness. Not sure where they've seen them, but they sound like a great space saving idea. Once the kids are awake they can tuck away one of the beds to free up some floor space.