Your kid's passports are perfectly valid. Even yours was. You're wasting your time packing the rest.
When is the last time you read that somone was denied boarding with a valid passport?
You're "keeping quiet" had nothing to do with it. Passsports are scanned. The agent barely looks at them. You breeezed through for the simple reasen your passport was not expiring before the end of the cruise
The 6 month rule for passport expiring is in the case you are flying to the country, not arriving on a ship. You don't need to be worrying. She has a valid passport up to the day coming home. Leave the birth certificate home