Our understanding is that Snapchat is not recommended for children under the age of 13 as younger children are vulnerable to exploitation when using this kind of social media application. Indeed Snapchat's own website has this statement: "The minimum age [for Snapchat] is 13, in compliance with theChildren's Online Privacy Protection Act (COPPA). But, when you download it, Snapchat asks for your date of birth, and—if your birth year tells it you're under 13 — you're redirected to the kid version,called “SnapKidz.”"
For further information and advice please see this link to the NSPCC’s guidelines for children and social media -https://www.nspcc.org.uk/preventing-abuse/keeping-children-safe/online-safety/