Night time wetting is not something kids can control. Don't pressure your kids! It only makes them feel guilty for something they can't help at all. My oldest two are 7 and 5. They wet the bed EVERY SINGLE NIGHT. We tried limiting their fluids at night (they don't drink after supper anyways), waking them up to go, etc. But it's just a matter of maturity. Their bodies are not producing the hormone that concentrates urine at night, and they are very deep sleepers. We've talked to the doctor about it, and my oldest will be seeing a bedwetting specialist in a couple of weeks, so we can try to do something before it becomes too embarrassing for her. But, if you have a look around online, you'll see it's more common for children to wet the bed regularly, some even into their teen years. (I really hope we're not headed for that!)
So, I sympathize with you and your frustrations. I dearly wish that my oldest two could be dry at night, as their three year old sister is! But, please try to remember that they really can't do anything about it! Put the nappy on, or a waterproof sheet down, or whatever works for you to keep the laundry to a minimum, and save the mattress, but don't make them feel guilty for it.
All the best!