I only have 2 kids as well but I think fortnightly is fine, I'm sure some families wash them only every 3 or 4 weeks. I wash the quilt covers & mattress protectors less frequently than that because we have top sheets and so that's the part of the bed that you're touching. If I had 2 more kids I would stagger the washes so that you're only doing 2 at a time - makes it a smaller (& less daunting) job. I also put clean sheets on straight after taking the others off, so it's not critical if I don't get the dirty ones washed & dried immediately. It means you need to have a few more sets of sheets but worth it IMO.