Hi, elaine. Check out the sandbox that Norm built on New Yankee Workshop. It doesn't use pressure treated wood in contact with the kids, the design requires no liner, and it has a top to keep cats out. The problem is that it requires Norm's shop to build it.
For what it's worth, if your kid is 5 years old now and you build it to last 10 years, how much do you expect your 15 year old kid will use it. Unless you plan on having a lot of kids, don't worry about a sandbox lasting over 5 years or so. That will give you an excuse to tear out the old one and plant something there, or build a gazebo.