We have a 7 year old DD, and a 16 month-old DD.
Up until now, we've all had dinner together, then DH, DD1 & I have a little treat after DD2 has gone to bed.
This isn't working so well, for 2 reasons. DD2 is going to bed later some nights, and I'd rather if we are having something sweetened, to have it WITH dinner (i.e. with protein).
However, if we eat it with dinner, even if I expend more effort to make sure it's always fruit based, wholegrain, lightly sweetened, etc., it means that DD2 is being exposed to dessert daily.
Separating them isn't really an option, it's a small, open house, and they are generally inseparable from when DD1 gets home from school until DD2 goes to bed.