SMS messages can be used to hack and unwanted messages could not be blocked.
If somebody had the number of the watch and your child could interact with a text, a hacker could get access or begin an unwanted conversation. It's for this that we decided to err on the side of caution and not allow SMS's to the watch altogether.
For the highest security, it's best for your trusted, close, Family Members to message over data using the Chat feature on your Angel Watch app. This allows you to securely communicate with the watch's Chat app and vice versa.
If your child wanted to communicate with others (that you wouldn't want to give access to your child's location), they could download Line Lite or WhatsApp on their device. This separates them entirely from any of the data on your Angel Watch app.