If you wish for Users to not utilize the button, In the Progress Text area via onBoard Settings > Progress Text, click the HTML button and at the very bottom enter something along the lines of:
 
<font size="3" color="red">Please do not use the +Apply Another Child button below. If you wish to apply another child, please start the process anew by going to <a href="schoolname.myschoolapp.com/app#login/apply">this page</a>.</font>

<font size="3" color="red">Please do not use the +Apply Another Child button below. If you wish to apply another child, please start the process anew by going to <a href="schoolname.myschoolapp.com/app#login/apply">this page</a>.</font>

Depending on how much you want it to stand out, you can increase the font size within the HTML code.