Don’t make a big fuss of telling them as if you were sitting them down to say someone has died or they are going to Disneyland.
Just mention it casually while you are doing other things and be prepared to answer all questions they may have.
The key thing is not telling them UNTIL you know what it is happening next exactly as it is the not knowing what makes the transition traumatic.
With DS we told him “as you have seen, we are not getting along very well so we thought it would be better for dad and I to live in different houses”
The questions went like this:
Am I going to see dad again? Yes, of course you will, what will happen is that you will have your own bedroom with your toys at your dad’s house, which is just near to xyz.
You will have special weekends with him every other weekend and will see him after school every Wednesday.
He also was concerned about missing his bed but as soon as he heard he was getting a bunk bed he wasn’t bothered. ExH made a lovely space for him in his new house so that helped.
ExH didn’t move out until he had the other house sorted enough for DS to go over there. I insisted he took DS to stay with him on the first night as that would prevent DS feeling left behind. It was all an adventure for him. The first years after divorce where wonderful for DS as he finally got his dad’s full attention.