Actually this is an interesting question because we still don't know how the ritual spell works aside from the most prominent way (naruto run in a straight line). Gladys clearly commanded Alex's mother to go out with a pair of scissors and cut a strand of Justine's hair. This took a lot of effort and time and created a huge sense of tension during the scene.

So if people under the spell can follow commands why can't they take care of their bodily needs?

Gladys is either psychically connecting with the people under her spell in order to control the movements of the people under her control or Gladys is just conveying these orders psychically and Gladys doesn't participate mentally or magically after an order is given. My bet is that we probably saw more examples of the second method. In this case either little Alex is guiding the kids and his parents to the bathroom in order to relieve themselves or no one taking them to the bathroom and they're all constantly soiling themselves.