My principal is a genius. I let her know that we could receive a referral sometime within the next 2-3 weeks, and she decided it was time for "the letter" to parents. She planned it well - it will be sent out next Thursday. We have no school on Friday (for semester grades) nor Monday (MLK Jr. Day).

So it will give parents Fri/Sat to receive and read the letter, adjust their kids to the news... and when we all return on Tuesday I can address it with the students myself. She advised me to focus on the positive adventure, and NEVER mention that I'm sad to leave or miss them (I told her not to worry about that!).
The letter announced succinctly that I was taking a child care leave expected in February, and then explained about the woman replacing me. Just very briefly mentioned at the end that "although we will miss her, we are supportive of her and her husband's desire to be home with their newly adopted infant." Smooth, that lady. Considering the last teacher to take leave was already 4 months pregnant and not married when the kids found out. :)

I just hope 153 sets of parents are as supportive as my principal!