There is a widespread custom to stand during the Friday night kiddush because it attests to the creation of the world, and witnesses must stand when giving testimony. The Arizal, basing himself on mystical considerations, also recommends standing; this is the custom of Sephardim and Ashkenazic Ĥasidim (SA 271:10; Kaf Ha-ĥayim 62). However, most Ashkenazic Jews sit during kiddush since it is supposed to be recited where one will be eating (see section 10 below), and meals are eaten sitting down. Additionally, sitting down together makes the audience and the person making kiddush into a clearly-defined group. There are some Ashkenazim who try to get the best of both worlds by standing during the recitation of Va-yekhulu and sitting down for the rest of kiddush (MB 271:46; SSK 47:28).
All agree that it is preferable to sit for kiddush during the day, and this is the common practice. Nevertheless, some are accustomed to stand, and this is not prohibited.
Even though a woman may make kiddush like a man, it is customary for the man to make kiddush for his family. When multiple families are eating together it is preferable that one person make kiddush for everyone, based on the principle that “In a multitude of people, the King is glorified” (SA 167:11).
It is customary to cover the challah during kiddush. Since bread is considered the more important food, normally if we have both wine and bread in front of us and we plan to eat from both, we recite the berakha on the bread first. But when making kiddush, the berakha on the wine must be made first. In order to avoid giving “incorrect” precedence to the wine, we cover the challah. Similarly, if there are mezonot (grain-based foods over which the berakha of Mezonot is recited), they should be covered during kiddush, as mezonot also normally take precedence over wine. When one is organizing a kiddush for the congregation after services, anyone intending to drink from the kiddush wine must cover any pastries in front of him. One who does not intend to drink from the wine after kiddush need not do so (See SSK ch. 47 n. 125). Based on this explanation, it is not necessary to have the challah on the table when one is making kiddush. If they are on the table, however, they must be covered.
Others provide an additional reason for covering the challah. The two Shabbat challahs allude to the manna that fell during desert times. The manna was covered above and below by layers of dew. To parallel this, the challah is placed above the tablecloth and covered. Accordingly, some people are careful to put the covered challah on the table before kiddush, as a reminder of the manna. Some leave the challah covered until after the recitation of “ha-motzi.” Some even cover the challah at se’uda shlishit for this reason (MB 271:41; AHS 271:22).