I guess if you have all the money and patience in the world to redo your bathroom every five years to keep up with the growing child that's fine.
Most people don't have that kind of money, time or patience though. I suggest designing the bathroom so that it can grow with the child. If you really want to get your rubber duckies in there maybe keep it with the shower curtain instead of the tile. :)
Below are 5 baths that I think would work great as Kid's Baths....or as Guest Baths once the kids are grown.