We're building a sandbox for our child...what's the safest way to treat it? Varnish okay? Paint? Thanks!?

If you are going to use treated lumber, go with the brown, not the green. Even though the green isn't as dangerous as it was before 2003, it still isn't something you want your kids chewing on when they get tired of building sand castles. Personally, I would go with regular lumber painted with fibergl resin. There is no fibergl in the resin, it is just the plastic you use when building with fibergl. Used as is, with just the hardener, it makes a nice shiny clear coat that will protect the wood, yet show off the natural grain. Or you can add pigment, and make it any color you want, and basically look like the same stuff the kiddy pool, or plastic slide set you have is made of.