Hi Ronda!
First of all, I see that you have a 6 day park pass. I would definitely recommend that you stay at your resort and not go into a theme park on the day of your arrival on Jan. 21. This will allow you and your kids time to rest up and not immediately hit the ground running. I would take the day to explore your resort and maybe even hit the pools & kids play areas if the weather permits.
I would encourage you to get to the parks as early as possible in the morning. The first few hours of each morning is a great time to get in a number of attractions with little or no wait. I would suggest around 11:30-12:00 to grab a bite to eat for lunch and then head back to your resort and let all the kids (and even the adults) take a nap and rest up. This will give everyone a chance to build up their energy for the evening hours in the parks. In the afternoon, I would head back to the parks between 4:00-5:00. I would encourage you to stay as late as your 3 children can take it.
I don't think you would need to utilize the Extra Magic Hours as much as you would if you were going during a more crowded time. The Morning Extra Magic Hours would be totally up to you. Getting to the designated park an hour early would probably be manageable, but with 3 children I wouldn't attempt the Evening Extra Magic Hours. I would be afraid of what it would do to your children the next day.
Please look at the following link to see the
Walt Disney World Theme Park Hours for the length of time during your visit. You will just need to update the date at the top with your arrival date. That site will also show you which parks will offer Extra Magic Hours on certain days.
I hope this helps you formulate a game plan for a wonderful and memorable Walt Disney World Vacation!
Have a magical trip!!
Scott