Nighttime accidents + wet mattress = unhappy parent. You can buy a plastic mattress protector, but as my grandma once said: "What? You want him growing up sleeping like he's in a cheap motel?"
She gave me this tip: put a wool blanket between the mattress and the fitted sheet. The blanket will stop any accidents from getting to the mattress. If an accident does happen, just spot wash the area with a dilute detergent solution and sun dry. I've bought blankets from Goodwill and the army supply store and cut them down to size (after I've washed them), but my favorite wool blanket supplier (they practice predator-friendly ranching – very cool) is Thirteen Mile Lamb and Wool Company.