There is a famous Halachic principle known as "En Kiddush Ela Be知akom Se置da," which means that one does not fulfill the obligation of Kiddush on Shabbat unless Kiddush is recited in the context of a meal. When one recites or hears Kiddush on Friday night or Shabbat morning, he does not fulfill the obligation of Kiddush unless he eats a Kezayit of bread or Mezonot food such as cake, or drinks a Rebi段t of wine. If a person hears Kiddush and does not eat or drink anything, or if he drinks something other than wine or eats foods like rice, fruits and vegetables, he has not fulfilled his obligation of Kiddush. And since he has not fulfilled the Misva of Kiddush, it turns out that he acted wrongly by eating, since one may not eat or drink on Friday night or Shabbat day before fulfilling the requirement of Kiddush.
Many synagogues have a Kiddush or breakfast for the congregants after the Shaharit or Musaf service on Shabbat morning. The Rabbi customarily recites Kiddush and drinks a Rebi段t of wine, and thereby fulfills his requirement of "Kiddush Be知akom Se置da." Others at the Kiddush will generally eat a Kezayit of bread, cake, crackers and the like, in which case they fulfill their obligation of Kiddush. Theoretically, they would not have to recite Kiddush again at all that Shabbat, since they fulfilled the Misva through the Kiddush in the synagogue. In practice, men usually recite Kiddush when they return home for lunch, on behalf of family members who had not recited Kiddush. But they personally have fulfilled their Kiddush obligation by listening to the Kiddush in the synagogue and then partaking of a Kezayit of bread or cake.
Some people, however, prefer not eating in the synagogue, and after hearing the Rabbi痴 Kiddush take a drink or eat some nuts or fruit. This is improper, as they do not fulfill the Kiddush obligation and have thus violated the prohibition of eating before Kiddush. Therefore, a person who at a Kiddush does not plan on eating a Kezayit of bread or cake and the like, or drinking a Rebi段t of wine, should not eat or drink at all. Furthermore, those who organize the Kiddush in the synagogue must ensure to provide enough cake, crackers and cookies for everyone to eat a Kezayit and thereby fulfill the Kiddush obligation.
It should be noted that Hacham Ovadia Yosef, in a famous responsum published in his Yabia Omer (vol. 2, Orah Haim 19), finds a possible basis for justifying the practice of taking a drink or small snack with Kiddush on Shabbat morning. He writes that this practice could perhaps be defended by combining two minority views. First, there is a view among the Halachic authorities that the rule of "En Kiddush Ela Be知akom Se置da" only applies on Friday night. On Shabbat day, according to this view, one does not have to drink wine or eat after Kiddush to fulfill the obligation. Furthermore, there is an opinion that if the person who recites Kiddush drinks a Rebi段t of wine, this suffices for everyone who heard Kiddush, and they therefore fulfill the obligation without any eating or drinking. Although we do not follow either of these opinions, Hacham Ovadia maintains that we may invoke them as a possible justification for those who do not eat or drink wine at a Kiddush. This practice is certainly wrong, but one should not protest against it, since there may be some justification for it.
Summary: A person who recites or hears Kiddush on Friday night or Shabbat morning must eat a Kezayit of bread, cake and the like, or drink a Rebi段t of wine, at that setting, or else he does not fulfill his Kiddush obligation. Therefore, a person who hears Kiddush should not eat or drink anything at the Kiddush unless he intends on eating a Kezayit of the foods mentioned or drinking a Rebi段t of wine.
