If you're planning of having a meal, have that as a combined activity. At the end of the meal, before the dessert is served, have the primary go down the their respective part of the church building and have them do their own activities while the adults stay where they are. Since the primary will have to have adult supervision, prearrange it so that those brothers and sisters will only have to stay down there for only half of the time and then be relieved by a second set of adults so not all will miss out on the adult part of the activity. The young women and young men can also help out with this. You may also be able to get another ward to help out here at this point and then help them out at another time. Towards the end of the activity, then you can serve desert either by bringing both groups back together or separately.