Hi Melissa! Given your children's ages, I would recommend visiting Magic Kingdom and Disney's Hollywood Studios more than once on your upcoming vacation. We always find that there are several attractions/shows that we miss at each Park on our first day, so it has been our tradition to visit the same Park on two days, so that we can enjoy the missed attractions/shows and repeat our favorites. It relaxes our visit to know that we do not have to get it all in on the first day, because we will return the next day.
So why Magic Kingdom and Disney's Hollywood Studios? I recommended them based on the number of attractions that your children would enjoy at their ages. When you factor in Advanced Dining Reservations and Character Meals, time truly goes by fast. Your family will enjoy being able to slow down and visit those attractions that interest you the most. But, we have been known to let the children decide which Park to revisit based on our trip planning discussions. I love the book Birnbaum's Walt Disney World for Kids as a great way to let our children help plan our vacation. This book allows them to get a glimpse into each Park and note the "must do" attractions and shows.
Our family usually takes a Resort Day in between our visits to the Parks. We will visit one Park on days 1 and 2, day 3 is a Resort Day, and visit another Park on days 4 and 5. If we are fortunate enough to stay longer, we will have another Resort Day on day 6. It is nice to have a relaxing day at the Resort to enjoy the activities offered at no additional cost during your stay. This includes fun pool activities, arts and crafts, and the arcade depending on the time of year we are visiting. Resort Day is essential when traveling with infants, toddler and pre-schoolers. Your children's ages may not require this, but it is a very nice option.
Have a Magical Vacation!