Hi @meyer2311. Just to clarify: You've set up a premium account through Facebook for your children to use. However, you don't want them to use the Facebook account to login, but instead a regular username and password, correct?
If I got that right, then on the confirmation email you received when you created your account you should see a username that's about 11 character of gibberish. Unfortunately, that is going to be your accounts username (with the Facebook password), and there's no way to change that.
Assuming I understood, that's going to be an issue because you don't want them to have access to the Facebook information (plus that username would be a pain to remember), so you'll probably want to contact support so they can "split" your account so it has a regular username and password. From there you'll be able to reconnect the account to Facebook, but you wont need Facebook to log in.
Support can be reached one of three ways: through the online contact form; on twitter by reaching out to @SpotifyCares; or by sending a message to the Spotify Cares Facebook page. Please be aware that responses may take anywhere from 24-48 hours depending on the volume of tickets support is currently handling.
In regards to the contact form, upon filling it out you should receive a confirmation email, usually a generic message containing the information you provided and instructions to visit these forums. If after 24 hours you do not hear back from a support team member, please reply directly back to the confirmation email, even if it originates from a no-reply address. If you do not receive this confirmation email, please check your spam folder. If it is not present there, please attempt using one of the other contact methods, or, if you are unable to use the alternatives, please let me know.