Make them take back the ice cream and bring a new one out with the food - they will. I've had them try to give me part of a meal while it sits on the counter waiting for the rest and I never accept it. They always have to give it to me altogether.
Drip feeding here but DD1 is a manager at McDs. She's only 18 herself and it brought home to me how much the place is run by teenagers. They don't always think of these things (crew turnover is pretty high) so point it out to them.
Since DD has told me about some of the (faintly ridiculous) complaints that reach her, which she always deals with politely and with a smile (possibly a grimace!), I've worried much less about complaining in places. If you do it in a polite, friendly way people seem happy to help.