People will generally think that yes, dinner is going to be served or offered. Because of the time frame of the party.
OR the invitation, should have... mentioned if it is only cake/ice cream, or dinner will be served.
That time slot, IS dinner time.
If you are going to serve dinner/food... have it out already and ready at 6:30. Then the kids can eat and play and whatever. BUT you then can do the cake/ice cream at say, 7:30.
You have only 2 hours to do the food/eating/cake/ice cream.
Thus, have the dinner/food/pizza out already and ready to eat, at 6:30. Don't wait. Kids will be, hungry at that time.