Keep in mind though, the adult 18+ can still be considered his dependents if they are currently taking education course full time up to age of 22 without any breaks continuously. If that applies to one of the adults, then they are to be added if the husband want to sponsor them in the future for Canadian PR.
If the children 18+ have no intention of becoming PR, then they do not need to be listed.
The 5669 background form, only your husband sign it. The background 5669 only applies to those sponsored for PR, in your case, the husband.
The children would have to fill out the background form themselves if they were being sponsored for PR but in this case, they are not. So not required of them.
It would be required of them to complete the form when the husband wants to sponsor them after the husband officially lands as PR.