I found this great product at walmart. It is a cup you keep in the freezer. You can fill it with orange juice and in a few minutes you have a slushie. I put the slushie in small bowls and the kids love it! You can find them in the seasonal section for about $13.00.
With food allergies they will sometimes show immediately and other times it can cause symptoms the next day so I would recommend like mickyc to keep a food log.