I.G.
For my potty trained child I use incontinence pads. Those are absorbent pads you can find in the depends aisle at the supermarket. They work great to catch the occasional accident and I "double up" as in layer a pad with a sheet then another pad with another sheet on top, so if she has a midnight accident, I just rip off one layer, throw the sheet in the washer and the pad in the trash.
Good luck.