The Aruch Hashulhan (Rabbi Yehiel Epstien, 1829-1908) writes that the Kiddush said before the morning Se’uda is for "Kabod Ha’Seuda"-to show honor and importance to the meal. The Rabbi of Brisk suggests that being that this is the reason for the Kiddush, everyone attending the meal should take a sip of the wine. Hacham Ovadia Yosef (Yehave Daat,Helek 5,page 89) argues that only the one making the Kiddush must drink the proper amount of wine, and it is not necessary for everyone at the meal to take a sip of the wine.
The Rama (Rabbi Moshe Isserles, 1520-1572) writes (Orah Haim 288, 7) that one should show that the morning Se’uda of Shabbat is different than a regular weekday meal. If one usually eats lunch late, on Shabbat he should have it early and vice versa.
The Halacha says that it is forbidden to eat or drink anything before Kiddush. The Shulchan Aruch (Orah Haim 289) writes that the obligation of Kiddush comes after Shahrit. Therefore, it will be permissible to drink something before Shahrit and this is not a problem of drinking before Kiddush.
Summary: It is not necessary for all those attending the Shabbat morning meal to taste the wine of the Kiddush. If one usually has his weekday meal late, on Shabbat he should have it early and vice versa. One is allowed to drink before Shahrit.
