We have an untreated pine bed that we bought at a furniture store across the street from Ikea when they were closing out. One of the slats had a knot in it and it broke. DH bought a piece of wood at Home Depot that was very similar and cut it to fit.
We had similar problems with our Ikea bed (stupid design). I fixed it by cutting a 2x4 the width of the mattress & screwing it to the inside of the side rails right in the center of the bed. This forced the side rails to pull in (instead of bowing out like they were doing). It made everything tighter & the slats no longer would fall through & move around.
You said you already nailed it? Try replacing the nails with screws - screws hold better - the squeaking you're getting could be the movement along the nails.