Hi
Obviously all schools will vary, but I would hope that most do it in a sensitive (and sensible) way.
I support two days a week in a very small Primary school, so our year groups are combined. Last year the class I was in combined R, Yr1, Yr2 which did work surprisingly well. R kids mostly played, but could 'sit in' on appropriate lessons if they wanted. There was not much difference between the less able Yr2s, and the more able Yr1s. Teachers always have to differentiate (or nearly always) within a class to meet different abilities anyway, so the mixed ages didn't really matter.
This year, due to staff changes, the combinations will be R, Yr1 in one class, and Yr2, 3, 4
all in another. The top class will be Yr5, 6.
So in your case I would guess, and hope, that Yr1 will start off with some formal lessons, probably in the mornings, but still retain some play/less formal activities for afternoons. Gradually the emphasis is likely to shift towards increased academic work, but afternoons might still be art, crafts, music, PE etc.
Hope all goes well for you both!