Have you learned exactly what the new rules are? For us, it's 6 days per semester before absences become an issue. With a clear understanding of the county's policy, you'll be better armed to go forward with your plans.
When notifying the school, I go straight to the teacher. I've never had a problem, although each teacher's reaction has been a little different as far as make up work is concerned. One teacher sent so much work, it was virtually impossible to get it all done during the trip. Last year the teacher was so excited for us because she is such a big Disney fan, she sent no school work with DD10. I think DD may have done a little extra at school/homework before/after the trip, but the actual trip itself was all ours and boy, was it nice!
I think there's a lot to be considered with this issue. First, I believe parents should be the primary educators of our children, the school supplements the efforts we put in at home. Then, I don't think perfect attendance is a reasonable goal to have. Whether sick or going on a trip, things happen. Why set your child up for the disappointment of a missed goal? I don't think perfect attendance at school will make a significant difference to the character of a child when they become an adult.
It seems more and more, school is becoming a prep course for standardized tests, rather than learning. With that in mind, I don't think an elementary student misses a whole lot, even if absent for an entire week.
We need to keep in mind there's lots of pressure on the teachers with the new "No Child Left Behind" program. Because of that, teachers may squirm a little as professionals, but on a personal level, I think most of them are thrilled that you're taking your child to the happiest place on earth!
I would notify the teacher as soon as possible, especially if you think your child will talk about the trip while at school. It's best the information come directly from you, rather than through chit chat, or gossip, etc.