Hi there,
I was just mattress shopping for my 2 1/2 year old yesterday! We are doing just like you--moving him into a full-size bed and using rails. I don't know if there are different types of rails, but I know at least some of them require a boxspring to use them. I'd rather have my son a little higher with rails than a little lower without. There is also such a thing as a low-profile boxspring. It's used by kids and others (sometimes older folks who don't want to have to climb so high to get in the bed). Also, some people use a bunkie board. Not sure if I spelled that right, but it's the thing you'd use to support the mattress on a bunkboard, and it's much thinner than a full boxspring. So those are options if you're worried about the height. There are also some bedframes (not sure what they're called) that you don't need to use a boxspring for, but I don't know much about that. If you're at all concerned about bedskirts, anything other than the traditional boxspring might make a bedskirt difficult or impossible. But not everyone uses them anyway. My theory is that I wanted to buy furniture, mattress included, that will last a long time, hopefully until my son is off to college and beyond. So we are going with a good mattress--not top of the line hugely expensive, but decent. Both places I looked mentioned one mattress in their stores as the "starter" mattress that most people get for the real little ones, and then the one they sell when they come back a few years later needing something else. We wanted to skip the starter mattress thing, especially because when they're so little, their weight is pretty insignificant and won't cause much weakening of the mattress anyway. I plan to put a good waterproof mattress pad on to eliminate the pee issue. One salesperson told us that it's been disproven that soft mattresses cause back problems--I have no idea if that's true. She also said kids tend to like softer mattresses. Again, that's her opinion. We went the middle route--not like a rock, but not the kind with the real plush top. If you're looking for a good quality mattress for less expensive than I've seen elsewhere, I highly recommend Magic Sleeper in Pottstown. We have our own king mattress from them, I've gotten a queen before this and my parents have always gotten their mattresses there. They have a website, but just so you know the Collegeville store is no longer open, just Pottstown. They make their own mattresses in the factory. We are probably buying my son's furniture at Feceras so I was thinking, we might as well get the mattress there. But it would have cost more for their starter mattress than for a good quality 15-year warranty mattress at Magic Sleeper. Plus, I don't know if you care about this, but all the major mattress manufacturers no longer make flippable matresses. This was important to me because I think that being able to flip the mattress makes it last longer. And for 50 bucks you can get your mattress made flippable at Magic Sleeper. So the price for the one we picked out (Chiro, not extra-soft or extra-firm, and NOT the starter mattress) would be $399 for the flippable mattress and boxspring. It would've been 50 dollars less for the non-flippable kind. And it has a 15-year warranty! At Feceras, it was over 600 for a non-flippable Serta that was considered their middle-of-the-road, with I believe only a 3-year warranty. And it was between 300 and 400 for their non-flippable starter mattress. So we're sold on Magic Sleeper.
Sorry so long but since I just went yesterday, I thought I'd give you the info I knew!
M.