I think the best time is when you are able to answer almost all obvious questions they have. You’re on your way to that in terms of where you’ll be living.
Ideally though you need to answer questions like “how often will we see dad? Where will we stay when with dad? Where do we go for Xmas? What about holidays? Etc etc….sit with stbex and try to preempt the questions they’ll have. And agree the ones you can, and come up with what you’ll say to one’s you don’t know yet.
one of worse thing you can do is tell them you’re separating/ divorcing and not be able to tell them what’s going to happen . The void, the unknown, is filled then with fear and anxiety. You, both of you, need to be telling them as much as possible about how it will impact their concerns, so there is no place for that void and then fear and anxiety to creep in
when you do tell them, do so together. Keep it short and simple- plan ahead what to say, and best you take a bit each so it’s not one of you saying it all. Something along lines of . “We have decided that it is best for us to live separately. That means living in separate homes and managing our day to day activities individually . We still love you both, and will always be your parents together and will both still be there to support you both “ Then pause, read their responses, give them time to process a bit, then ask them if they have more questions, or want some time for a while to think . Offer cuddles. Be positive, smile, reassure, do NOT cry or do anything particularly emotional- you both need to be solid rocks to support them . They’ll need time over the next hour to think of the immediate questions. Give them that, but if they want to go off give them that too. Say to them any time they have questions, worries, or just want to talk to ask you,
you don’t need to say about divorce. You don’t need to discuss finances or the legal side of child custody. Do not at any time , even in future years, discuss the causes of the breakdown of marriage. Even courts don’t need to know that now.
id say tell them on a Saturday morning. Don’t tell in evening or late afternoon as it’ll just mess their sleep . Don’t say during week- they won’t be able to ask question the next day. Saturday morning gives them time during the day and then Sunday. Try to do something to take their minds off it by late Saturday afternoon, some thing familiar and normal. Same on Sunday - get into normal Sunday afternoon/evening routine. Reinforce that life will go on for them and things can be normal.
it may be worth, if you feel you could do so, to mention it to someone in school on Monday - just in case of wobbles. But they do need to go to school on Monday even if they’re upset. Best thing for them to start to process.
remeber kids go through the grief/change pathway too. If you’re not familiar read up on google. They’ll have all those same feelings of denial, bargaining, anger, before coming to terms and accepting their new reality. The more you can help them understand those feelings are normal, and help them control the things they can control, the better.