When we explain our situation in this way -- to our kids and to other people -- it seems to be generally well-received: "We want to do what is best for each child, individually, for each year of his/her education." This shows that we are open-minded, not bashing any other educational options, and in a continuing process of seeking out our best paths. Homeschooling is the right fit for our children right now, but that is not to say we won't ever consider other options.
I think your kids would be appreciative if you are straightforward with them. I remember distinctly, near the end of 1981 (!!), when my 2nd grade teacher took me into another room and explained to me that I was going to be moved up to the 3rd grade classroom. It wasn't a radical acceleration, like going to college when I was 10 or anything, but still very unusual for our small private school. I understood my teacher's seriousness, and her explanation that this was in my best interest, and her concern that they didn't want me to be upset (socially) by the move. I was excited and a little nervous about the transition, but it was handled really well (with a little farewell party from the 2nd grade class and a welcome from the 3rd graders) and I had the peace of knowing it was a decision made jointly by the teachers and my parents, for my own good.
The immediate, instinctual idea of permanency can be frightening for kids, so you could present the idea of homeschooling along these lines: "Are you sick of being bored at school everyday? Would you like to try homeschooling for a year, or even half a year, and see how we all like it? You would probably learn a lot more in a lot less time, and have more time to pursue things that YOU are interested in -- academic subjects, hobbies, jobs, sports..."
