Hi Congrats!
Well, like some other people here, I told mine before we got the visas, a week or so after we applied. I told my dad first and he said he would tell my mum when the time was right. I didn't feel that was a cop out, as he would be able to break it to her more gently, rather than me go down and make a big announcement. At that stage we didn't know whether we would get visas or not. My dad was shocked, but said we had to go for it, in fact he said he nearly emigrated to Canada about 40 years ago but didn't because of his mum. He advised us not to make the same mistake.
He told my mum a week or so later and while she was upset she could see it could be a very good move for us. I still feel guilty though as while they have a lot of friends, I worry about their health as they get older. We told them last September and got the visas (163) at beginning of June, so heading out in 3 weeks.
Good luck, you may find that while they may well be emotional, they will understand and support you.
All the best
Ross