Kids Halloween costumes in Shanghai are most easily bought on Taobao, or, you can use Baopals in English – they even have a special Halloween page.
If you prefer to browse some shops for kids Halloween costumes in Shanghai, then you can try:
Ruby Party: Room 9, F, Hongqiao Zhidi Building, 201 Chengjiaqiao Zhi Lu
Neon Kids: 10 Pu’an Lu near Jinling Lu
The market by the Science & Technology Museum, 2000 Century Avenue, by Yingchun Lu (In Metro Line 2 Shanghai Science & Technology Museum Station)
Jajaja Party: 158 Biyun Lu near Baihua Lu