May one bring food to the table prior to Kiddush?
Some learn that one is to refrain from bringing any of the foods to the table prior to Kiddush, unless they are covered just like the Chalas. However from Admur it is clear that it is only necessary to cover the bread and not the other foods, and so is the widespread custom.
Must one place Challah on the table before Kiddush?
The custom is to place Challah on the table before Kiddush and to cover it.
 Peri Megadim 271 A”A 7; See Piskeiy Teshuvos 271 footnote 184
 The reason: As one of the reasons behind covering the bread is to show that the food was brought to the table for the sake of Shabbos [i.e. Yikra Deashabbasa]. Initially the custom was not to bring the food at all to the table prior to Kiddush due to this [see Admur 271/17; Tur 271; Tosafus Pesachim 100b; Sheilasos Yisro 54; Rashbam Pesachim ibid], and now that we bring it to the table, at the very least one must cover it, and there is no difference between the Challah and the other foods. [P”M ibid]
 So is clear from the concluding wording of Admur ibid that “The custom is to bring it initially before Kiddush and cover the bread until after Kiddush”; See Hearos Ubiurim Tzemach Tzedek 7/257
The reason: Perhaps the reason for this is because the main aspect of the meal is the bread, and only by it is applicable the concept of Yikra Deshabbaso. Alternatively, perhaps it is a Tircha to cover all the foods and hence it is not done.
 Piskeiy Teshuvos ibid
 Admur 271/17“The custom is to bring it initially before Kiddush and cover the bread until after Kiddush”; Aruch Hashulchan 271/22; Minchas Yitzchak 3/13-10; Piskeiy Teshuvos 271 footnote 184
Other customs: Some are accustomed not to place the Challas on the table until after Kiddush. [Maaseh Rav 118]
 The reason: In order to diminish in making an interval between Kiddush and the meal. [See Admur ibid] Alternatively this should be done in memory of the Mun. [Aruch Hashulchan ibid; mentioned in Admur ibid]