Family Share question

i I have sent two invites to my children and it says safari can not open invite bc it is an invalid address, not sure what to do. Also currently we use one account am I understanding correct that I will need to set them both up Apple ID's to use family share? Lastly I do not own a credit care ia there a way to set up an account for a child under 13 with my debit or any other way?

Thank you!