You Can take screenshots on Snapchat without notifying by clearing cache and storage. Make sure you remember your account details because you will have to login in again. Now, follow the given below steps to take screenshots on Snapchat without them knowing.
- Open your Snapchat and load the snap(Don't watch).
- Turn on airplane mode.
- Now, watch the snap and take a screenshot of the Snap.
- Then, Go to settings-->Apps & Notifications -->Snapchat-->Force stop.
- Hold the Snapchat icon and go to the app settings-->Apps & Notification-->Snapchat-->Stroage-->Clear cache.
For more read - https://onlinegeeks.net/unique-and-cool-snapchat-usernames-ideas/