I don't know anything about toddlers needing soft mattresses. Baby mattresses aren't that soft.
At any rate, DS1 moved into a bed at 3. (I don't think it makes sense to move them before you have to, as they'll just climb out of bed and generally cause trouble.) He went straight into a slat bed with futon, exactly the same as our bed, only narrower.
I think you mean a futon on the floor, though, right? I don't see why not. Well, the only thing would be dust - I've slept on the floor before, for ages, and if you're not very good about dusting etc, the futon gets dusty. I think they get aired better on a slat bed.
(With a bed rail, they don't fall out, and of course the longer you wait before moving to a bed, the less likely he will be to fall out.)
Thanks everyone. Yes it is a mattress on the floor I was thinking of, rather than the slatted base.
Tiffini, did your ds take to a bed easily, or did he get up to mischief?
I don't know why I'm even thinking about this, the cot is no problem...
MowJo - DD had no probelms adjusting to a bed; infact she slept better in a bed than in a cot. When she went in we had no issues of her getting in and out at all, or about er falling out. After some excitiemnt at Christmas and before (at about 20mo) she did get in and out a bit, but we did CC for 3 nights and it was sorted (and not bed related).
My ds moved into a bed at 12mths but thats because he was doing comando vault attempts over the side of the cot. I only had a problem with the bed for first couple of nights as ds realised he could get on and off the bed...now he doesnt move at all until the morning. Dont see why a futon would be a problem at all.
My dd hated her cot so moved to a matress on the floor at 18mths, purely for the safety factor of not falling out. She loved it, felt very grown up and bedtimes became magical. She wakes up early hours of morn and climbs into our bed, but we're happy with that.
At 2 she moved into a proper bed and has never fallen out. And once in it, she only gets up to request more milk or if she needs the toilet.