Sabine her kids speak primarily Italian, they are living in the area where Neapolitan is spoken but Sabine is German. Sabine talks frequently in both Italian and German to her kids and when she gets angry she turns to Neapolitan, it can be really expressive .. :)
Now what is the mother tongue of Sabine's kids ? They speak primarily Italian...
In WiktionaryZ we have people that indicate that their mother tongue is zho or Chinese. According to Ethnologue Chinese is a macrolanguage. This implies that Chinese cannot be a mother tongue, one of the 13 languages Chinese is divided in can only be the mother tongue. This is a potential hot potato when people equate Chinese with the country and not the language.
WiktionaryZ is about languages and only about languages.
It can also be understood differently, they may mean that Chinese is the first written language that they learned. However if I understand things well, when people talk about the Chinese written language, it is actually Mandarin. For Yue for instance, there is a need for additional characters that are in one of the later versions of the UNICODE. This is however not what I would consider a mother tongue. A mother tongue is the language that you learned from your mother. Writing is what you learn at school.