As an autism mom, and basically any mom, I'm grateful there are mattress protectors for my cute little bed wetters so my whole house doesn't smell like urine. The best mattress protectors I have found are at Ikea. They are really soft on the top and have that vinyl-type bottom to keep the wetness off the mattress. The sides that go over the bed are made of a stretchy fabric, so they don't rip if your kids are...say...bed jumpers.
My sister, also known as the coolest person ever at our house, also got these great mattress covers like the ones you use at the hospital, for my kids since they are getting bigger and sometimes mattress protectors don't fully protect the mattress.