My mum gave me this idea. I have 5 children and 4 of them were sleeping on hard mattresses where you could feel the springs. Over the years I have accumulated double duvets (not sure if you call them the same in the US, perhaps divans? Any way the things you put covers on to sleep under, you know what I mean!)
As I couldn't afford to replace the mattresses, my mum suggested I fold the double duvets in two and lay them over each mattress (in effect a nice thick mattress topper), then make the beds as usual. It has worked a treat, no expense, just used stuff I had stored in the roof . My kids all get a better night sleep now!