其实我们还只是个孩子
[qí shí wŏ men hái zhĭ shì gè hái zi]
Implying a sense of youth and immaturity, despite all the grown-up experiences or appearances. The nickname might indicate a longing for innocence or an excuse for childish behaviors due to still being 'young at heart'. In this case the word 'child' could be more about mentality rather than actual age.