Bed slats that slide out of position are annoying if not dangerous and can create a bunk bed disaster. Add friction to the wooden slats by wrapping rubber bands around the ends of the slats where they contact the bed rails. This will keep them in place yet still allow the slats to be easily removed for repositioning the bed or for cleaning.
Rubber band solution for slat movement has a problem. Rubber bands have a short life. A couple of years and they have dry rotted or whatever. Cut up a small tire intertube for a longer life solution.