Hi Karen,
Thank you so much for visiting the Disney Parks Moms Panel with your question. It is wonderful that you have a "bonus" day at the Walt Disney World theme parks, and that it will allow you to head to a theme park right when you arrive at Walt Disney World!
Which theme park you decide to visit on that first night might depend on where your resort is. You might want to visit a theme park that is close to your resort so that you can get to the theme park quickly. If you are staying at a resort with monorail access or at Disney's Fort Wilderness Resort or Disney's Wilderness Lodge, you could probably get to Magic Kingdom the quickest. If you are at Disney's Beach or Yacht Club or Disney's BoardWalk Inn, you can get to Epcot or Disney's Hollywood Studios rather quickly by boat or by foot. At other resorts, you can ask a cast member which theme park is the closest if you are concerned about getting to a theme park quickly.
If you are not concerned about how long it will take to get to the theme park, I would suggest Magic Kingdom. Your little ones will probably be so excited on their arrival day, and seeing Cinderella Castle on that first day will be magical. Magic Kingdom is beautiful at night, and you should be able to catch both
Main Street Electrical Parade and
Wishes Nighttime Spectacular. These amazing nighttime shows will set the mood for a wonderful vacation.
I hope that these thoughts are helpful for you, Karen. Have a wonderful time planning your vacation, and please do not hesitate to ask if you have any further questions.