DS1 is 6, in year 1, and sits on a table of 4 children, the mother of one of whom (a little girl, N) has just died of stomach cancer over the weekend.
The little girl is likely to return to school tomorrow.
I was thinking of saying something like "N's mummy has died. It is very sad because she was not old. Everything will feel very different at home for N. So at school she will probably want things to be the same as usual. So she may not want to talk about it. If she does want to talk about it that's ok too. It's not something that you can make better, but remember that it's even more important to be kind and friendly."
It doesn't need to be perfect, just age-appropriate and to give DS1 the sense that he has to follow the girl's lead.