First of all, my mother always said, "you gotta do what's right for you." So you really need to do what you are most comfortable with!
My personal view is this ... be honest. You don't want to set an example for your child that says you can lie when it makes things "easier." Also, a 5-6 year old child is going to want to talk about the trip! How will you feel if you give the teacher an excuse and then he/she learns the real reason? Makes trust a real concern! As a teacher, I was always excited (and jealous!) when my students would be out of school for an exciting trip! We would work together to be sure the child did not fall behind. (I do know that not all teachers are the same about this.) My view was that all experiences benefit a child and this one has such a wealth of things to see and do; what a great source of "background experiences" which are so good for children and truly contribute to learning!!!!
Have fun!!!!
JenC in MD