Allowed Contacts - SMS when device locked

I'd like my children to have the ability to send SMS text messages to "Allowed Contacts" while their phone is locked.  There are situations where they may not be able to call for help or may receive social pressure if they speak to a parent/emergency contact about getting out of certain situations.  Texting the Allowed Contact ensures the message is sent without my child having to speak the message out loud in a group or emergency situation.