Anyways I have found a great trick to help protect the mattress when you start putting your kids in underwear at night. Puppy pee pads!
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I figured it out when I was potty training Hazel and I had some left over from when our dog had puppies. I just put a couple under the sheets so she didn't know they were there. If she had an accident I would have to wash the sheets but the mattress would be safe and I would just throw the pad away and put down a new one. And they are so much cheaper than any of the stuff they have specifically for children's beds.