[PHXFlyer]

Delta has seats which it will not assign, and as a matter of fact telephone agents cannot assign, until the day of departure. There are also a number of seats which are reserved for elite members which are made available to anyone when the 24 hour online check-in window opens. Because of the fact that you are Australian nationals originating in Australia I'm not sure if online check-in will be an option for you but at exactly 24 hours before the flight it can't hurt to try. Otherwise do as others have said and arrive at the airport extra early. Those seat blocks for airport assignment are done so with this very scenario in mind.

Sometimes travel agencies and resellers who sell large blocks of seats aren't even allowed to submit seat requests until the entire block is sold and the passenger list is finalized. This may be what happened with Expedia.au since they do have access to some of these "bulk" fares and perhaps you bought one of these unknowingly. If you shopped around and Expedia was selling tickets for less than what was available for the same flights directly through Delta it was probably a bulk fare. Expedia's explanation of why the seat assignments were not transmitted to Delta was therefore flawed. It may not have been an error but that the seat requests for the entire block they sold were transmitted at the same time and you and your daughter were separated at that point. Expedia could have done a better job of explaining that to you however when you talk to a call center you don't know who you are talking to or where.

Finally just this advice. In your original post you talked of booking your tickets and seats. Actually you book your tickets but seats assignments are a request and not guaranteed. Most of the time they do not change but there are situations, such as aircraft substitutions, when they do. It's a good idea, especially if you book far in advance, to check your reservation online periodically to ensure that your flight times and/or seat assignments haven't changed. I'm traveling next week on a three-segment itinerary booked about 5 months ago. United substituted the aircraft on the first of the three segments and there is no longer a first-class cabin. The flight is just under an hour so it's really not a huge deal that I'm no longer in first class but when the change happened I was randomly assigned a new seat which was a window which was not my preference so I was able to change my seat to an aisle before all of the other aisle seats were assigned. Remember in the end you a buying a seat from point A to point B but not a specific seat on the aircraft that get you there.