It is best to teach them how to look for public restrooms at an early age. This way, when they are grown up, they will have that skill well ingrained. You might want to look for the public restrooms every time you go into a store or restaurant. This way when your grandchild really has to go, they will know that there is a restroom, and where to look for it, or how to understand the directions.
With boys, you have to teach restroom etiquette because of the difference between how men and women conduct themselves there.