This can be done by clicking My Dashboard > Messages > Send Message > Scroll down and click Sent Text above the subject line.
Parents have to be connected to their Scouts to show up and Scouts need to be invited by their parents to show up for messages and email. See our Getting a unit started guide for more information on setting them up: https://help.scoutbook.scouting.org/knowledge-base/getting-a-unit-started-in-scoutbook/
Only those who have setup text messaging can receive text messages.