If your child's account is paired to your own, you can change their password from your Parent Account.
If you haven't already, this article will walk you through linking your child to you: How to Link a Child's Account to a Parent Account
Here's how to change their password:
1.Go to Prodigy and select "Login" from the top right portion of the page.
2.Enter the e-mail address you created your Parent Account with and your password, then select Log in. If your account was created with Google, select that option.
3. Click View Progress under the name of the child you'd like to change the password for (or View > on mobile).
4a. Select Manage Child from the left side of the page.
4b. On mobile, select More in the bottom-right, then Manage.
5. You'll see your child’s login information displayed here. Enter a new password in the blank password field and select Save.
You have now changed your child's password!