I never added a admin user to my daughters MacBook Air but it is asking for one

When my daughter and I were setting up her laptop we never put an administrators username or password.Now all of a sudden its asking for it.Everytime she want to download an app it ask for it and we cant change the password because it needs the admins password.There is also an admins user and we never added that.What should we do?

[Re-Titled by Moderator]