Can anyone recommend a nice but not ridiculously expensive mattress for a child? My 3.5 year old's new bed is getting delivered tomorrow and I need a mattress and low profile box spring to go with it. DH thinks the mattress we got our eldest is too cheap and uncomfortable (I think that while it's not comfy when he lays in it, for her 35-lb frame, it is plenty supportive, but whatever!) and wants to do better with this purchase than we found d last time.

Would love to hear specific brands, models, and places to buy online if you remember! Thanks!