Paint the bathroom in a color that will be appealing to kids but still support an antique theme. Pale pastels might work for a girls' bathroom, since it will go well with some of the antique elements you'd use for a girls' bathroom. Dark blue, red or green might work well for a bathroom for boys, depending upon the items you want to add to the bathroom. A boys' bathroom decorated with antique toy fire trucks, for instance, might look better in dark red.
Furnish the bathroom with antiques that provide some functionality as well as visual appeal. A claw-foot tub and antique toothbrush and soap holders featuring cartoon characters, for example, can be used for visual appeal as well as functionality. Place an old wooden bench stool near the sink for kids who may need to climb up to access something on a higher shelf.
Hang framed vintage cartoons on the walls, and set antique character tins on the counter for holding assorted items. Go with a theme if your children have a particular interest, such as superheroes, Bugs Bunny or Mighty Mouse.
Set an antique toy box in the bathroom for storing towels and washcloths. The toy box can not only be used for storage. Your kids can use it as a place to sit when they get out of the bathtub. Hang shelves on the wall and display a collection of rubber ducks from the past, old miniature cars, superhero and cartoon collectibles, or a porcelain doll collection.