I didn't go for a big announcement with mine (3 and 5). DH did a few trips to the country we are now in, so we talked about it, Skyped him, chatted about what he was doing, looked at it on the map, then I came out for a visit, by which point they were quite interested, and so I said, next time I go, you'll come with me.
And then we gradually talked about how we'd stay for a long time, new school, new friends, then coming back to the UK for visits, someone else looking after our house while we are away, family visiting in their holidays etc.
So it was a much more gradual process. I think children are quite accepting of things - if you make it a big thing, with a big announcement, they are more likely to react.
If you introduce the idea of the new country, then work up to introducing the move as a more permanent arrangement, and mention each change that will happen individually as it arises in conversation/life - eg when you buy school bits here, talk about the 'new' school uniform etc - it is much easier for them to process.
I waited until they asked about the school, new country etc, so were either interested or wanting reassurance, before showing them specific info like the school brochure and website. But I thought it was better to be led by them asking than me presenting lots of hard info about their new life and expecting them to automatically be delighted.
But having all the positive arguments lined up ready is good, as they tended to ask questions when I wasn't expecting it! So they were clearly turning it over in their minds. But I did also balance those with the reality - 'so granny won't be up the road anymore but she'll come on a plane for her holidays!' etc.
Good luck!