Hi Amanda,
Extra Magic Hours are a great perk for guests of Walt Disney World resorts. My family and I often take advantage of them - especially the extra hour in the morning. We find that getting into the theme park an hour earlier than many people is a great perk.
You will need your Walt Disney World resort room key, called your Key to the World card, to attend Extra Magic Hours. This is true for everyone in your party, including children. Everyone in your party, even your children, will be issued a Key to the World card upon check in at your resort. To gain access to Extra Magic Hours, you will need the Key to the World Card for every member of your group, including children, to show that you are staying at a Walt Disney World resort.
For Extra Magic Hours in the morning, you will show your Key to the World cards at the turnstiles. This way anyone who is allowed access to the theme park has proven that they are staying at a Walt Disney World resort. For evening Extra Magic Hours, you may be asked to present your Key to the World cards to gain access to attractions. Guests without a Key to the World card are then asked to leave when it is evening Extra Magic Hours. Do not worry, Disney does not clear the theme park and make you re-enter for evening Extra Magic Hours. You just may be asked to produce your Key to the World card in the parks at that time.
Be sure to check the schedule of Walt Disney World park hours for which theme park has Extra Magic Hours on which day. You can also consult the Times Guide you receive when you check into your resort for information on Extra Magic Hours.
I hope that this helps explain Extra Magic Hours for you, Amanda. Have a great trip, and please do not hesitate to come back and ask if you have any further questions!