I have a family of five, and two of our phones experienced this glitch when updating to iOS 12. I call it the "Frankenstein-chat". What happens when the phone is affected by this glitch, it takes all phone numbers and emails that are linked to iMessage on your Apple ID and merges them into one Frankenstein-chat.
In your case, it took the iMessage chat with your daughter and iMessage chat with your husband and made them one. It will only show up under one of their names at a time, so it won't look like a group message. Very deceiving.
To fix this, simply identify this Frankenstein-chat on the main menu of the Messages app (it should display either your daughter's name or your husband's name). Once found, slide the chat from right to left to reveal a Delete button. Delete this faulty chat and then send a message to your daughter and husband's contacts separately as if they're new people. They should be effectively separated by now, assuming that neither of their phones were also affected by this glitch.
*be sure to save any precious photos or media that you don't want to lose when deleting the old chat in Messages
Hope this helps!