Since you have booked a package, your tickets and dining plan will be coded onto your room keys (KTTW cards). You will not be able to get your tickets at MGM; they have to be printed at WL upon check-in.
Your DH should be able to get the KTTW cards for your whole family with his ID.
If your room is not ready, he will be allowed to do a "pre-check-in", wherein he will get the KTTW cards, but they will not assign a room number to you; when a room becomes available, WL will assign it to you and your KTTW cards will work on the door. Your DH will be given a phone number to call after check-in time to find out your room number.
WDW resort room keys are plastic cards called Key to the World (KTTW) cards. KTTW cards can be used for a lot of stuff; they are first and foremost your room key, but you can also have park passes and dining plan encoded on them, and you can also have them enabled to charge purchases to your room. Each person, including the kids, will get their own KTTW card. You can enable room charging on each card seperately, however, in case you don't want your younger kids having that priviledge.
The KTTW cards themselves hold no information on the magnetic strip other than the cards serial number, so if you lose one, all you have to do is go to the front desk at WL, show your ID, and have a new card printed. Information about how much of your park pass is used, what items were charged, how many dining plan options you have used, etc. is all stored in the WDW computer system, not on the card itself.