I am actually having a hard time picturing someone going "oooh, we got an invite for a birthday party! Oh, it's just for cake and ice cream. Nevermind." lol.
I think by communicating that upfront (not because cake and ice cream isn't adequate, but just in case someone might have assumed there would be more, they could be sure to get everyone fed and happy beforehand) you're all set.
The only party that I really kind of depend on them serving food (and they do) is one where we have to travel an hour and a half to get there, plus their parties last hours (like 4). So 7 hours with party and travel is way too much to not have food.