Hello XP72,
Thank you for using Apple Support Communities! I understand that your child is still able to use Snapchat on an iPhone during Downtime while all other apps are blocked as expected. Being able to manage Screen Time settings for your family is important, and you've come to the right place for help.
Let's start by ensuring that the child's iPhone has been updated to the latest iOS version. If you do need to update iOS, test if the issue still occurs after:
Update your iPhone, iPad, or iPod touch
If you're managing the Screen Time settings from your device, go to Settings > Screen Time, select your child's name. Under Always Allowed, ensure that Snapchat is not added to the allowed apps list. Go to Downtime on the previous screen and ensure that Block at Downtime is turned on. Keep us posted on what happens after trying those steps.
Cheers!