Hi, Joe. Railroad ties are not good and neither is pressure treated wood. They both have chemicals that are not good for kids. Cedar is the best choice.
You might try to catch the New Yankee Workshop episode featuring a sandbox. The design is pretty good but the awning is a little overdone. And I always thought that a sandbox should be covered to keep cats and other critters out. You might check out some of the books with kids toys at the home center or library for plans. Also, you might look for plans on the web. I did check Jim Barry's site, my usual source for plans, but there were none listed there. I don't know how you build, but I usually don't slavishly follow plans as much as check them for features that I want to incorporate into what I build. For example, Norm's has a good design for keeping the sand in and letting the water out.