Two children are sitting on a white blanket, while the other child is sitting in the dirt.
both of the children are sitting in dirt
All the children cannot be sitting in dirt if two of them are sitting on a white blanket.
There are either 3 children, with one sitting in the dirt, or there are two children, both in the dirt.
If one of the children is sitting on a white blanket then both of the children cannot be sitting in dirt.