It's going back over 5 years, and if I'm honest, I think I have blanked a lot of it from my mind because it was so traumatic, but I think we told them only a few days before I moved out. I don't know if that was the right thing or not, but our thinking was that since we're throwing such a huge shock at them, it was better to follow up very quickly with the actual separation so that they didn't start to feel it wasn't going to happen and then have to go through it all again on moving out day.
Like I say, I'm not sure if that's the "right" way. Just our way. I was the one that left, so I am sure my exW would give a different perspective. I know how much it hurt the girls, and my eldest in particular. She went on to have real problems with eating and panic attacks. We did actually get her some time with a counsellor to help her talk things through.
One of the positive and "exciting" things they got to do was to get themselves settled into their new room at my new place. I gave them carte blanche to set it up how they wanted, which really seemed to help them feel grounded in the new environment. 5 years later, I have a bigger place so they have their own rooms, and have decorated and sorted them out how they want them.
My work means that our childcare arrangement was for me to only have them alternate weekends, and a number of weeks through the year during holidays so as much as I hate it, they don't really see my place as "home", more like "dad's". Be prepared for that... I also made sure that I paid more than enough to their mum each month so that they could stay in the house they'd grown up in even though it meant I had ( and still have ) a smaller place. I realise I'm fortunate enough that my job enabled this, so that may not be an option.
Be watchful of their behaviour. They're young so they might not be able to articulate how they are feeling so show it in their actions and behaviour. My eldest projected a huge amount of anger onto the lady I left my wife for. It was me that she was angry at, but she was scared if she showed it she would push me away. The counselling helped with that, as did my constant reassurance to her that no matter what she said or felt, I would still love her.
If you need any indication of how hard this is on the pair of you ( though infinitely more so on the kids ), even though I have moved on in every way from my marriage and relationship of over 20 years, I actually have tears in my eyes as I'm writing this because I remember how much it hurt them and seeing their faces on the day I told them.
For that reason, and I'm sure you've already considered this... I'd just say this to you. Are you both TOTALLY sure ?
Sorry, that's way too long for a response... Just started typing and remembered a pile of stuff :)
RLD.