I got online from Amazon, waterproof bed pads.
Just put in the search word "waterproof bed pads" or "waterproof crib pads" and many will come up. They have many... I got 4 of them. I like the "quilted" types... because it absorbs nicely. It is fabric... not plastic or vinyl.
I put the pads, DIRECTLY under my kids. (Not under the fitted sheet) DIRECTLY under my kids. If there is a pee accident, its real easy. I just remove it, put it in the laundry basket, put another one under them and that's it. And it can be put in the washer like anything else and put in the dryer.
The bedding/sheets do not get soiled. Lot less headache and hassle.
My kids are 7 and 3.5 and I use it daily. It really protects the bed.
All the best,
Susan