T.D.
Don't know anything about IKEA's beds, but I saw a nice looking white one at WalMart for $40. You would need to buy a mattress separately, but at least the sheet sets would be easy to buy. You could also check resale shops, Kid to Kid and Once Upon a Child have nice quality things and have locations all over the metroplex.
I wouldn't think a foam mattress would last very long or provide enough support once they start weighing more.
~ t