You want a solid wood frame and as another poster has said cheap slats will fuck a good matress.
My matress cost £500 i was realy loathe to pay that much tbh, but i have crap back.
6 months on and it hasnt lost any shape or spring.
If you are buying for kids, once they are over 12 they will need an adult matress and not a cheap £30/40 one.
It is an expensive thing to buy, but they are desperate for your money, get haggling and they might throw in the kids single's in with your own double.
Nothing to lose by trying mate