Just Put Them!
Tissue papers and toilet papers sound and look like identical twins. They are often interchanged by people, and that is understandable, but not when it defeats their purpose. Tissue papers are mainly used for faces and hands, while toilet papers are for restroom purposes. Also, tissues are packed in bundles while toilet ones are rolled.
It is safe to assume that this family knew this thing, but they were not just in the mood to visit the grocery this time. As a result, they decided to put their tissue in the restroom instead. After all, they are still called papers, right?