Hi maryann879,
Though MyAT&T online you would be able to see the phone number, date and time any messages were sent from a phone on the account historically. However messages are not able to be read.
However it should be known that any lines that are using iMessage or any other types of internet based messaging (such as Whatsapp, GroupMe, etc) will not show up on your bill as a billed text message or any information (such as date and time).