Just use the ground rule. Always. Complain it's otherwise too difficult for the children to learn. Then, in a few years time, a spellings reform will say putting an s behind the word is ok. (Except for names that are derived from plants or animals, or some other silly exception).

<rant>Dutch spelling reforms are great. It makes that the longer ago an author wrote his book, the harder it is to read. And hence, Dutch people don't quote Vondel like the British do with Shakespeare. (In fact, for Dutch people, it's easier to read Shakespeare than Vondel)</rant>