There are many reasons why children in Shanghai can't speak Mandarin. First of all, Shanghai is a city dominated by dialects, and Shanghai dialect is the local accent of Shanghai. Most Shanghainese can speak Mandarin, but they can't. Secondly, Shanghai's education system has also influenced the situation that Shanghai children speak Mandarin. Generally speaking, Shanghai schools pay more attention to students' Shanghai dialect than Mandarin, so many Shanghai children can speak Shanghai dialect instead of Mandarin. Finally, there are great differences between Shanghai dialect and Mandarin in pronunciation, intonation and grammar. It is more difficult for Shanghai children to learn Mandarin than Shanghai dialect, so many children are unwilling to learn Mandarin. In short, there are many reasons why children in Shanghai can't speak Mandarin. However, if the opportunity to learn Mandarin is improved, the level of Mandarin of Shanghai children will also improve.