Just a little insight...my mom just retired after teaching elementary school for 16 years, my husband taught jr/sr. high for 16 years and I taught college for many years as a tenured professor and am certified to teach in 7-12 and have worked in administration in h.s.for a number of yes....my answer...DO IT and don't look back.
Unless your child is struggling considerably...it will all work out. My mom always says that kids who have parents who are involved with them and in their education typically tend to do fine. If you are even concerned about taking them out of school then I would say you fall into this category.
Besides, a lot of people are home schooling these days. I haven't heard any of them saying they don't take trips because their children will miss the work that they would have done at home. A lot of learning can take place on trips of this nature, depending on the age. Let the teacher know you are going and then ask what they would like you to do while you are gone in regards to school work. In elementary school many teachers will give you some worksheets and alot won't bother to give you anything. That being said, if they do give you work, please make sure you have your child do it. Consider the time they spend in pulling this work together and be considerate of that. It always irrated my mom when she put together work for the child and the it was never returned.
High school gets a bit trickier but WDW is very educational at the age especially in the area of history, geography, culture and science. Lots of things that can be done to "compensate" for the time that they have lost. At this age, students can even come up with some suggestions that would align closely with what the are studying.
I am a firm believer in family time. Kids are only home for a short amount of time and then they are all grown up. This year I am evaluating charter schools for my son and one of the questions I ask is in regards to missing time from school. So far, all of them say, have a great time (and by the way do you want to take me?). GO FOR IT and never look back.