In the instance of multiple characters of varying heights, like in the example below, you would select the medium height and place the horizon line there.
Here, you have the dog as the lowest point and the two adults at the high point. The boy is between the two, so the horizon line is placed there.
This is not an absolute rule and can be changed to the adult's level or the dog's level if you wanted to.
In the two drawings below, the character on the left has their feet drawn on the same perspective plane as the surface plane. The character on the right has his feet turned so they're almost parallel to each other and flat to the viewer. This creates the illusion that the character is floating off the surface plane.
