The standard we usually see is some type of white tile flooring. Since this is a boy's bath, I would use a dark wood looking tile with matching grout so it would be easier to keep clean. Hope your boys don't do like the son of one of our clients. White and blue bath (Duke blue) including a white tile floor and white subway tile on walls. The 12 year old son had a paint ball fight with a friend in the bath a couple months after we finished. Ruined lots of stuff.